git-gui: Allow 'git gui version' outside of a repository

git-gui: Allow 'git gui version' outside of a repository

I got a little surprise one day when I tried to run 'git gui version'
outside of a Git repository to determine what version of git-gui was
installed on that system.  Turns out we were doing the repository
check long before we got around to command line argument handling.

We now look to see if the only argument we have been given is
'version' or '--version', and if so, print out the version and
exit immediately; long before we consider looking at the Git
version or working directory.  This way users can still get to
the git-gui version number even if Git's version cannot be read.

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