git-gui: Handle gitgui tags in version gen.

git-gui: Handle gitgui tags in version gen.

I've decided to use gitgui-0.5 as the format for tags in the
git-gui repository.  The prefix of gitgui was chosen here to
make its namespace different from the namespace used by git
itself, allowing developers to pull both tag namespaces into
the same repository.

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