git-gui: Prefer version file over git-describe.

git-gui: Prefer version file over git-describe.

Some distributions are using Git for part of their package
management system, but unpack Git's own source code for
delivery from the .tar.gz.  This means that when we walk
up the directory tree with git-describe to locate a Git
repository, the repository we find is for the distribution
and *not* for git-gui.  Consequently any tag we might find
there is bogus and does not apply to us.

In this case the version file should always exist and be
readable, as the packager is working from the released
.tar.gz sources.  So we should always favor the version
file over anything git-describe guess for us.

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