Introduce BaseBranchHelper around candidate resolution

GUI call sites that need a base branch all share the same logic: ask
GetBaseBranchCandidates, take candidates[0] as the config-order
tiebreak, and surface the candidate list when the user needs to
disambiguate. Extracting that into a helper keeps the upcoming prompt
and rebase wiring focused on UX concerns. Not yet routed through —
subsequent commits replace the direct GetBaseBranchCandidates calls
with ResolveBaseBranch.

pkg/gui/controllers.go

@@ -129,6 +129,7 @@ func (gui *Gui) resetHelpersAndControllers() {
 		Search:     searchHelper,
 		Worktree:   worktreeHelper,
 		SubCommits: helpers.NewSubCommitsHelper(helperCommon, refreshHelper),
+		BaseBranch: helpers.NewBaseBranchHelper(helperCommon),
 	}
 
 	gui.CustomCommandsClient = custom_commands.NewClient(

pkg/gui/controllers/helpers/base_branch_helper.go

@@ -0,0 +1,37 @@
+package helpers
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/commands/models"
+)
+
+// BaseBranchHelper resolves the base branch for a given branch. The
+// closeness rule (smallest ahead value) usually picks a single answer
+// but can leave a tie when the branch's fork point is reachable from
+// more than one main branch — in that case the helper surfaces the
+// candidates so the caller can disambiguate.
+type BaseBranchHelper struct {
+	c *HelperCommon
+}
+
+func NewBaseBranchHelper(c *HelperCommon) *BaseBranchHelper {
+	return &BaseBranchHelper{c: c}
+}
+
+// ResolveBaseBranch returns the base branch for the given branch, the
+// full set of tied candidates (for any disambiguation UI), and whether
+// the answer is genuinely ambiguous (more than one candidate tied at
+// the closest position).
+//
+// An empty baseRef (with no error) means no configured main branch
+// contains the branch — not an error condition.
+func (self *BaseBranchHelper) ResolveBaseBranch(branch *models.Branch) (baseRef string, ambiguous bool, candidates []string, err error) {
+	mainBranches := self.c.Model().MainBranches
+	candidates, err = self.c.Git().Loaders.BranchLoader.GetBaseBranchCandidates(branch, mainBranches)
+	if err != nil {
+		return "", false, nil, err
+	}
+	if len(candidates) == 0 {
+		return "", false, nil, nil
+	}
+	return candidates[0], len(candidates) > 1, candidates, nil
+}

pkg/gui/controllers/helpers/helpers.go

@@ -53,6 +53,7 @@ type Helpers struct {
 	Search            *SearchHelper
 	Worktree          *WorktreeHelper
 	SubCommits        *SubCommitsHelper
+	BaseBranch        *BaseBranchHelper
 }
 
 func NewStubHelpers() *Helpers {
@@ -90,5 +91,6 @@ func NewStubHelpers() *Helpers {
 		Search:            &SearchHelper{},
 		Worktree:          &WorktreeHelper{},
 		SubCommits:        &SubCommitsHelper{},
+		BaseBranch:        &BaseBranchHelper{},
 	}
 }