Change GIT-VERSION-GEN to call git commands with "git" not "git-".

Change GIT-VERSION-GEN to call git commands with "git" not "git-".

GIT-VERSION-GEN can incorrectly return a default version of
"v1.3.GIT" because it tries to execute git commands using the
"git-cmd" format that expects all git commands to be in the $PATH.
Convert these to  "git cmd" format so that a proper answer is
returned even when the git commands have been moved out of the
$PATH and into a $gitexecdir.

Signed-off-by: Junio C Hamano <junkio@cox.net>