Show update-ref commands in rebase todo list

This is useful when working with stacked branches, because you can now move
"pick" entries across an update-ref command and you can tell exactly which
branch the commit will end up in.

It's also useful to spot situations where the --update-refs option didn't work
as desired. For example, if you duplicate a branch and want to rebase only one
of the branches but not the other (maybe for testing); if you have
rebase.updateRefs=true in your git config, then rebasing one branch will move
the other branch along. To solve this we'll have to introduce a way to delete
the update-ref entry (maybe by hitting backspace?); this is out of scope for
this PR, so for now users will have to type "git rebase --edit-todo" into the
custom command prompt to sort this out.

We will also have to prevent users from trying to turn update-ref commands into
other commands like "pick" or "drop"; we'll do this later in this branch.

pkg/commands/git_commands/commit_loader.go

@@ -311,7 +311,9 @@ func (self *CommitLoader) getInteractiveRebasingCommits() ([]*models.Commit, err
 	}
 
 	for _, t := range todos {
-		if t.Commit == "" {
+		if t.Command == todo.UpdateRef {
+			t.Msg = strings.TrimPrefix(t.Ref, "refs/heads/")
+		} else if t.Commit == "" {
 			// Command does not have a commit associated, skip
 			continue
 		}