GIT-VERSION-GEN: adjust for ancient git

When an ancient "git" that does not understand "describe"
command is on the $PATH, "git describe" emitted a Usage message
without exiting non-zero status (which is a mistake we cannot
fix retroactively).  Catch this case to make sure we do not try
using phoney multi-line string as a version number.

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

GIT-VERSION-GEN

@@ -3,9 +3,17 @@
 GVF=GIT-VERSION-FILE
 DEF_VER=v1.4.2.GIT
 
+LF='
+'
+
 # First try git-describe, then see if there is a version file
 # (included in release tarballs), then default
-if VN=$(git describe --abbrev=4 HEAD 2>/dev/null); then
+if VN=$(git describe --abbrev=4 HEAD 2>/dev/null) &&
+   case "$VN" in
+   *$LF*) (exit 1) ;;
+   v[0-9]*) : happy ;;
+   esac
+then
 	VN=$(echo "$VN" | sed -e 's/-/./g');
 elif test -f version
 then