git-gui: Refactor current branch menu items to make i18n easier

git-gui: Refactor current branch menu items to make i18n easier

The i18n team has also identified a rather ugly block of code in
git-gui that is used to make a pair of Repository menu items show
the current branch name.  This code is difficult to convert to use
[mc ...] to lookup the translation, so I'm refactoring it into a
procedure.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>