When somebody else extracts git tarball inside a larger project,
'git describe' would reported the version number of that upper
level project.

Sometimes, using the consistent versioning across subdirectories
of a larger project is useful, but it may not always be the
right thing to do.

This changes the script to check ./vertion file first, and then
fall back to "git describe".  This way, by default, tarball
distribution will get our own version.  If the upper level wants
to use consistent versioning across its subdirectories, its
Makefile can overwrite ./version file to force whatever version
number they want to give us before descending into us.
Signed-off-by: NJunio C Hamano <junkio@cox.net>