Add a function to find the branches stacked below a branch

lazygit will offer to push these along with the current branch, so that
a rebased stack can be pushed in one go. The commits panel already marks
the tips of stacked branches, and this uses the same criterion. A branch
is below the given one if its tip is one of the loaded commits of that
branch that isn't merged into a main branch. This needs no git command.
In whole-graph mode the commit list also contains commits of other
branches, but those are marked as merged, so they don't count.

Not used yet.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

pkg/gui/controllers/helpers/branch_stack.go

@@ -0,0 +1,39 @@
+package helpers
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/commands/models"
+	"github.com/samber/lo"
+)
+
+// Returns the branches stacked below the given branch: those whose tip is one
+// of the given commits of the branch that hasn't been merged into a main
+// branch yet. The branch itself and the main branches are left out. The
+// result is in the order in which the tips appear in commits, so the branch
+// closest to the given one comes first.
+//
+// Only the commits passed in are looked at, so a branch whose tip is further
+// down the history than the loaded commits is not found.
+func BranchesBelowInStack(
+	commits []*models.Commit,
+	branches []*models.Branch,
+	branch *models.Branch,
+	mainBranches []string,
+) []*models.Branch {
+	branchesByTip := map[string][]*models.Branch{}
+	for _, other := range branches {
+		if other.Name == branch.Name || lo.Contains(mainBranches, other.Name) {
+			continue
+		}
+		branchesByTip[other.CommitHash] = append(branchesByTip[other.CommitHash], other)
+	}
+
+	result := []*models.Branch{}
+	for _, commit := range commits {
+		if commit.IsTODO() || commit.Status == models.StatusMerged {
+			continue
+		}
+		result = append(result, branchesByTip[commit.Hash()]...)
+	}
+
+	return result
+}

pkg/gui/controllers/helpers/branch_stack_test.go

@@ -0,0 +1,108 @@
+package helpers
+
+import (
+	"testing"
+
+	"github.com/jesseduffield/lazygit/pkg/commands/models"
+	"github.com/jesseduffield/lazygit/pkg/utils"
+	"github.com/samber/lo"
+	"github.com/stefanhaller/git-todo-parser/todo"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBranchesBelowInStack(t *testing.T) {
+	hashPool := &utils.StringPool{}
+	commit := func(hash string, status models.CommitStatus) *models.Commit {
+		return models.NewCommit(hashPool, models.NewCommitOpts{Hash: hash, Status: status})
+	}
+	todoCommit := func(hash string) *models.Commit {
+		return models.NewCommit(hashPool, models.NewCommitOpts{Hash: hash, Action: todo.Pick, Status: models.StatusRebasing})
+	}
+	branch := func(name string, tip string) *models.Branch {
+		return &models.Branch{Name: name, CommitHash: tip}
+	}
+
+	current := branch("current", "c3")
+	mainBranches := []string{"master", "main"}
+
+	scenarios := []struct {
+		testName string
+		commits  []*models.Commit
+		branches []*models.Branch
+		expected []string
+	}{
+		{
+			testName: "stack of branches, closest to the current one first",
+			commits: []*models.Commit{
+				commit("c3", models.StatusUnpushed),
+				commit("c2", models.StatusPushed),
+				commit("c1", models.StatusUnpushed),
+				commit("m1", models.StatusMerged),
+			},
+			branches: []*models.Branch{current, branch("first", "c1"), branch("second", "c2"), branch("master", "m1")},
+			expected: []string{"second", "first"},
+		},
+		{
+			testName: "branch pointing at the same commit as the current one",
+			commits: []*models.Commit{
+				commit("c3", models.StatusUnpushed),
+				commit("m1", models.StatusMerged),
+			},
+			branches: []*models.Branch{current, branch("twin", "c3")},
+			expected: []string{"twin"},
+		},
+		{
+			testName: "several branches pointing at one commit keep their order",
+			commits: []*models.Commit{
+				commit("c3", models.StatusUnpushed),
+				commit("c1", models.StatusUnpushed),
+			},
+			branches: []*models.Branch{current, branch("one", "c1"), branch("two", "c1")},
+			expected: []string{"one", "two"},
+		},
+		{
+			testName: "main branches are left out even if their tip isn't merged",
+			commits: []*models.Commit{
+				commit("c3", models.StatusUnpushed),
+				commit("c1", models.StatusUnpushed),
+			},
+			branches: []*models.Branch{current, branch("main", "c1")},
+			expected: []string{},
+		},
+		{
+			testName: "branches whose tip is merged are left out",
+			commits: []*models.Commit{
+				commit("c3", models.StatusUnpushed),
+				commit("m1", models.StatusMerged),
+				commit("m0", models.StatusMerged),
+			},
+			branches: []*models.Branch{current, branch("old", "m0"), branch("master", "m1")},
+			expected: []string{},
+		},
+		{
+			testName: "branches whose tip is not among the commits are left out",
+			commits: []*models.Commit{
+				commit("c3", models.StatusUnpushed),
+			},
+			branches: []*models.Branch{current, branch("sibling", "s1")},
+			expected: []string{},
+		},
+		{
+			testName: "todo commits of a rebase don't count",
+			commits: []*models.Commit{
+				todoCommit("t1"),
+				commit("c3", models.StatusUnpushed),
+			},
+			branches: []*models.Branch{current, branch("other", "t1")},
+			expected: []string{},
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.testName, func(t *testing.T) {
+			result := BranchesBelowInStack(s.commits, s.branches, current, mainBranches)
+			names := lo.Map(result, func(b *models.Branch, _ int) string { return b.Name })
+			assert.Equal(t, s.expected, names)
+		})
+	}
+}