Add disambiguation menu to BaseBranchHelper

When ResolveBaseBranch reports a tie, callers need a way to ask the
user which of the configured main branches to use as the base.
ShowPicker takes the candidate list and a continuation, presents a
menu of short branch names, and runs the continuation with the
user's selection. Subsequent commits wire this into the three GUI
actions that care (rebase-onto-base, view-divergence, move-commits).

pkg/gui/controllers/helpers/base_branch_helper.go

@@ -2,6 +2,9 @@ package helpers
 
 import (
 	"github.com/jesseduffield/lazygit/pkg/commands/models"
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/utils"
+	"github.com/samber/lo"
 )
 
 // BaseBranchHelper resolves the base branch for a given branch. The
@@ -35,3 +38,26 @@ func (self *BaseBranchHelper) ResolveBaseBranch(branch *models.Branch) (baseRef
 	}
 	return candidates[0], len(candidates) > 1, candidates, nil
 }
+
+// ShowPicker presents a menu of candidate base branches and runs
+// onPicked with the user's selection. Callers should only invoke this
+// when ResolveBaseBranch reported ambiguous=true; for the
+// single-candidate case there is nothing to pick.
+func (self *BaseBranchHelper) ShowPicker(
+	branch *models.Branch,
+	candidates []string,
+	onPicked func(baseRef string) error,
+) error {
+	items := lo.Map(candidates, func(ref string, _ int) *types.MenuItem {
+		return &types.MenuItem{
+			Label:   ShortBranchName(ref),
+			OnPress: func() error { return onPicked(ref) },
+		}
+	})
+	return self.c.Menu(types.CreateMenuOptions{
+		Title: utils.ResolvePlaceholderString(self.c.Tr.PickBaseBranchTitle,
+			map[string]string{"branchName": branch.Name}),
+		Prompt: self.c.Tr.PickBaseBranchPrompt,
+		Items:  items,
+	})
+}

pkg/i18n/english.go

@@ -563,6 +563,8 @@ type TranslationSet struct {
 	ViewDivergenceFromUpstream            string
 	ViewDivergenceFromBaseBranch          string
 	CouldNotDetermineBaseBranch           string
+	PickBaseBranchTitle                   string
+	PickBaseBranchPrompt                  string
 	DivergenceSectionHeaderLocal          string
 	DivergenceSectionHeaderRemote         string
 	ViewUpstreamResetOptions              string
@@ -1715,6 +1717,8 @@ func EnglishTranslationSet() *TranslationSet {
 		ViewDivergenceFromUpstream:           "View divergence from upstream",
 		ViewDivergenceFromBaseBranch:         "View divergence from base branch ({{.baseBranch}})",
 		CouldNotDetermineBaseBranch:          "Couldn't determine base branch",
+		PickBaseBranchTitle:                  "Pick a base branch for {{.branchName}}",
+		PickBaseBranchPrompt:                 "More than one configured main branch is a candidate for this branch's base. Pick which one to treat as its base.",
 		DivergenceSectionHeaderLocal:         "Local",
 		DivergenceSectionHeaderRemote:        "Remote",
 		ViewUpstreamResetOptions:             "Reset checked-out branch onto {{.upstream}}",