Make GIT-VERSION-GEN tolerate missing git describe command

Make GIT-VERSION-GEN tolerate missing git describe command

I think it is probably a bug that "git non_existent_command"
returns its error message to stdout without an error, where
"git-non_existent_command" behaves differently and does return an
error.

Older versions of git did not implement "git describe"  and
GIT-VERSION-GEN produces an empty version string if run on
a system with such a git installed.  The consequence
is that "make rpm" fails.

This patch fixes GIT-VERSION-GEN so that it works in the
absence of a working "git describe"

Signed-off-by: John Ellson <ellson@research.att.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>