git-gui: Ensure version number is always current.

git-gui: Ensure version number is always current.

I'm stealing the exact logic used by core Git within its own Makefile to
setup the version number within scripts and executables.  This way we
can be sure that the version number is always updated after a commit,
and that the version number also reflects when it is coming from a dirty
working directory (and is thus pretty worthless).

I've cleaned up some of the version display code in the about dialog too.
There were simply too many blank lines in the bottom section where we
showed the version data.

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