Generalize CanDoFastForwardMerge into IsAncestor

The function asks git whether HEAD is an ancestor of a ref, and its name
says what the one caller wants to know. A later commit asks the same
question about a branch that isn't checked out, so let the caller name
both refs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/commands/git_commands/branch.go

@@ -285,11 +285,12 @@ func (self *BranchCommands) Merge(branchName string, variant MergeVariant) error
 	return self.cmd.New(cmdArgs).Run()
 }
 
-// Returns whether refName can be fast-forward merged into the current branch
-func (self *BranchCommands) CanDoFastForwardMerge(refName string) bool {
+// Returns whether the first ref is an ancestor of the second one, which also
+// means that the second one can be fast-forward merged into the first one
+func (self *BranchCommands) IsAncestor(ancestorRefName string, refName string) bool {
 	cmdArgs := NewGitCmd("merge-base").
 		Arg("--is-ancestor").
-		Arg("HEAD", refName).
+		Arg(ancestorRefName, refName).
 		ToArgv()
 	err := self.cmd.New(cmdArgs).DontLog().Run()
 	return err == nil

pkg/gui/controllers/helpers/merge_and_rebase_helper.go

@@ -525,7 +525,7 @@ func (self *MergeAndRebaseHelper) MergeRefIntoCheckedOutBranch(refName string) e
 	}
 
 	wantFastForward, wantNonFastForward := self.fastForwardMergeUserPreference()
-	canFastForward := self.c.Git().Branch.CanDoFastForwardMerge(refName)
+	canFastForward := self.c.Git().Branch.IsAncestor("HEAD", refName)
 
 	var firstRegularMergeItem *types.MenuItem
 	var secondRegularMergeItem *types.MenuItem