branch split: prefill names with branch prefix (#1126)

## Problem

branch split does not currently respect the configured branch prefix.
Unfortunately, there are issues with applying the branch prefix
to branch split unconditionally,
or even as an implicit post-processing step,
as that leaves the current branch name unusable for reuse.

Reusing the current branch name is already a meaningful split operation:
it means "move this existing branch to this split point."
If `feature` is the current branch and `prefix/` is configured,
then always adding the prefix after input changes the meaning:

    current branch: feature
    typed name:     feature
    final name:     prefix/feature

That creates or selects `prefix/feature`
instead of referring to the current branch `feature`.

It's confusing to add prefixing for all branches
except the current branch, with or without exceptions like,
"only if it doesn't already start with the prefix."

## Solution

Rather than deal with confusing or ambiguous behavior,
we seed the interactive branch-name input
with the configured prefix.

With `prefix/` configured, the prompt starts as:

    Branch name: prefix/[cursor]

The prefix is normal editable text.
Users can accept or extend it for new prefixed branches,
or delete it and type the current branch name when they want to reuse
it.

`--at` remains literal because it already spells out the exact branch
name to use:

    gs branch split --at HEAD~2:feature

Because, again, we want to allow reuse of the current branch name
without prefixing.

Resolves #1119

.changes/unreleased/Changed-20260425-101922.yaml

@@ -0,0 +1,3 @@
+kind: Changed
+body: 'branch split: Branch name prompts now start with the configured branch creation prefix.'
+time: 2026-04-25T10:19:22.267463-07:00

branch_split.go

@@ -20,6 +20,8 @@ type branchSplitCmd struct {
 	split.Options
 
 	Branch string `placeholder:"NAME" help:"Branch to split commits of."`
+
+	Prefix string `default:"" config:"branchCreate.prefix" hidden:""`
 }
 
 func (*branchSplitCmd) Help() string {
@@ -151,6 +153,7 @@ func (cmd *branchSplitCmd) Run(
 			fields := make([]ui.Field, 0, len(selected)+1) // +1 for deferred HEAD field
 			theme := view.Theme()
 			for i, commit := range selected {
+				branchNames[i] = cmd.Prefix
 				desc := cmd.commitDescription(theme, commit, false /* head */)
 				input := branchNameWidget(desc, &branchNames[i])
 				fields = append(fields, input)
@@ -171,6 +174,7 @@ func (cmd *branchSplitCmd) Run(
 
 				desc := cmd.commitDescription(theme, headCommit, true /* head */) +
 					" [" + cmd.Branch + "]"
+				headBranchName = cmd.Prefix
 				return branchNameWidget(desc, &headBranchName)
 			}))
 

doc/includes/cli-reference.md

@@ -828,6 +828,8 @@ would require a new name to be provided for commit 3.
 * `--at=COMMIT:NAME,...`: Commits to split the branch at.
 * `--branch=NAME`: Branch to split commits of.
 
+**Configuration**: [spice.branchCreate.prefix](/cli/config.md#spicebranchcreateprefix)
+
 ### git-spice branch squash {#gs-branch-squash}
 
 ```

doc/tapes/20260426-branch-split-prefix-prompt.gif

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:d0c084185b05810ed4612584ebaddd4ee4d0f7a7bdb8beb38a5e6a71ee97e9a7
+size 153506

doc/tapes/20260426-branch-split-prefix-prompt.tape

@@ -0,0 +1,48 @@
+# Demo of 'branch split' pre-filling names with the configured prefix.
+
+Output "20260426-branch-split-prefix-prompt.gif"
+Set Shell "bash"
+Set FontSize 16
+Set Width 640
+Set Height 420
+Set Padding 10
+Set CursorBlink false
+
+Hide
+Type "cd $(mktemp -d) && git init && git commit --allow-empty -m 'Initial commit'" Enter
+Type "alias gs=git-spice" Enter
+Type "gs repo init" Enter Sleep 100ms
+Type "mkdir stardrive && touch stardrive/chimes && git add stardrive && gs branch create stardrive -m 'Tune warp chimes'" Enter Sleep 100ms
+Type "touch stardrive/glitter-nebula && git add stardrive/glitter-nebula && gs commit create -m 'Chart glitter nebula'" Enter Sleep 100ms
+Type "touch stardrive/comet-tea && git add stardrive/comet-tea && gs commit create -m 'Brew comet tea'" Enter Sleep 100ms
+Type "git config spice.branchCreate.prefix 'fleet/'" Enter Sleep 100ms
+Type "clear" Enter Sleep 100ms
+Show
+
+Sleep 500ms
+
+Type "gs ll" Enter
+Sleep 2s
+
+Type "gs branch split" Enter
+Sleep 2s
+
+Enter
+Sleep 800ms
+Enter
+Sleep 800ms
+Enter
+Sleep 1s
+
+Type "warp-chimes"
+Sleep 500ms
+Enter
+Sleep 1s
+
+Type "nebula-chart"
+Sleep 500ms
+Enter
+Sleep 1s
+
+Type "gs ll" Enter
+Sleep 3s

testdata/help/branch_split.txt

@@ -43,3 +43,6 @@ Global Flags:
   -v, --verbose        Enable verbose output ($GIT_SPICE_VERBOSE)
   -C, --dir=DIR        Change to DIR before doing anything
       --[no-]prompt    Whether to prompt for missing information
+
+Configuration (🔧):
+  spice.branchCreate.prefix

testdata/script/branch_split_prefix_at_literal.txt

@@ -0,0 +1,46 @@
+# branch split leaves --at names literal when a branch prefix is configured.
+#
+# https://github.com/abhinav/git-spice/issues/1119
+
+as 'Test <test@example.com>'
+at '2024-06-23T09:50:12Z'
+
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+gs repo init
+
+git add feature1.txt
+gs bc features -m 'Add feature1'
+
+git add feature2.txt
+gs cc -m 'Add feature2'
+
+git add feature3.txt
+gs cc -m 'Add feature3'
+
+git config spice.branchCreate.prefix 'myuser/'
+
+gs branch split --at HEAD~2:feature1 --at HEAD^:feature2
+
+gs ls -a
+cmp stderr $WORK/golden/after.txt
+git graph --branches
+cmp stdout $WORK/golden/graph.txt
+
+-- repo/feature1.txt --
+feature1
+-- repo/feature2.txt --
+feature2
+-- repo/feature3.txt --
+feature3
+-- golden/after.txt --
+    ┏━■ features ◀
+  ┏━┻□ feature2
+┏━┻□ feature1
+main
+-- golden/graph.txt --
+* 99827e4 (HEAD -> features) Add feature3
+* a1192a4 (feature2) Add feature2
+* b9014d5 (feature1) Add feature1
+* 5c2596e (main) Initial commit

testdata/script/branch_split_prefix_prompt.txt

@@ -0,0 +1,62 @@
+# branch split prompts with the configured branch creation prefix.
+#
+# https://github.com/abhinav/git-spice/issues/1119
+
+as 'Test <test@example.com>'
+at '2024-06-23T10:00:12Z'
+
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+gs repo init
+
+git add feature1.txt
+gs bc -m 'Add feature1' features
+git add feature2.txt
+gs cc -m 'Add feature2'
+git add feature3.txt
+gs cc -m 'Add feature3'
+
+git config spice.branchCreate.prefix 'myuser/'
+
+env ROBOT_INPUT=$WORK/robot.golden ROBOT_OUTPUT=$WORK/robot.actual
+gs branch split
+cmp $WORK/robot.actual $WORK/robot.golden
+
+gs ll -a
+cmp stderr $WORK/golden/after.txt
+
+-- repo/feature1.txt --
+feature1
+-- repo/feature2.txt --
+feature2
+-- repo/feature3.txt --
+feature3
+-- golden/after.txt --
+    ┏━■ features ◀
+    ┃   1b17c72 Add feature3 (now)
+  ┏━┻□ myuser/feature2
+  ┃    0c7ed55 Add feature2 (now)
+┏━┻□ myuser/feature1
+┃    a3a4c59 Add feature1 (now)
+main
+-- robot.golden --
+===
+> Select commits: 
+> ▶   a3a4c59 Add feature1 (now)
+>     0c7ed55 Add feature2 (now)
+>   ■ 1b17c72 Add feature3 (now) [features]
+>   Done
+> Select commits to split the branch at
+[
+  "a3a4c59",
+  "0c7ed55"
+]
+===
+> Branch name: myuser/ 
+>   □ a3a4c59 Add feature1 (now)
+"myuser/feature1"
+===
+> Branch name: myuser/ 
+>   □ 0c7ed55 Add feature2 (now)
+"myuser/feature2"

testdata/script/branch_split_prefix_reuse_name.txt

@@ -0,0 +1,44 @@
+# branch split leaves --at names literal,
+# including intentional reuse of the original branch name.
+
+as 'Test <test@example.com>'
+at '2024-06-23T10:00:12Z'
+
+cd repo
+git init
+git commit --allow-empty -m 'Initial commit'
+gs repo init
+
+git add feature1.txt
+gs bc -m 'Add feature1' features
+git add feature2.txt
+gs cc -m 'Add feature2'
+git add feature3.txt
+gs cc -m 'Add feature3'
+
+git config spice.branchCreate.prefix 'myuser/'
+
+gs branch split --at a3a4c59:features --at 0c7ed55:feature2 --at 1b17c72:feature3
+
+gs ll -a
+cmp stderr $WORK/golden/after.txt
+
+git branch --show-current
+cmp stdout $WORK/golden/current_branch.txt
+
+-- repo/feature1.txt --
+feature1
+-- repo/feature2.txt --
+feature2
+-- repo/feature3.txt --
+feature3
+-- golden/after.txt --
+    ┏━■ feature3 ◀
+    ┃   1b17c72 Add feature3 (now)
+  ┏━┻□ feature2
+  ┃    0c7ed55 Add feature2 (now)
+┏━┻□ features
+┃    a3a4c59 Add feature1 (now)
+main
+-- golden/current_branch.txt --
+feature3