git-gui: Generate a version file on demand.

git-gui: Generate a version file on demand.

Because git-gui is being shipped as a subproject of the main
Git project and will often have a different lifecycle than
the main Git project, we should ship our own version number
in the release tarball rather than relying on the main Git
version file.

Git's master Makefile will invoke our own with the target
dist-version, asking us to save off our GITGUI_VERSION value
into our own version file, so that our GIT-VERSION-GEN script
can recover it at build time.

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