git-gui: Added repack database menu option, to invoke git repack.

git-gui: Added repack database menu option, to invoke git repack.

Most users of git-gui probably shouldn't be invoking git repack directly;
instead we should be looking at how many loose objects they have and
how many active packs they have and making the decision for them.  But
that's more work to code, and there's always going to be discussion about
what is the right default threshold and how do we know that the user is
willing to do the repack when we decide its time, etc.

So instead we'll just keep it simple and offer up the menu option.

Unfortunately right now we get now progress indication back from
git-pack-objects as its being invoked not through a tty, which makes
it disable progress output and the git-repack.sh wrapper won't let us
pass through --progress.

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