GIT-VERSION-GEN: restrict tags used

Restrict the tags used to generate the version string to those that
begin with "v", since git's tags for git-core (ie. excluding git-gui)
are all of the form "vX.Y...".

This is to avoid using private tags by the user in a clone of the git
code repository, which may break certain machinery (eg. Makefile, gitk).
Signed-off-by: NTay Ray Chuan <rctay89@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

GIT-VERSION-GEN

@@ -12,7 +12,7 @@ if test -f version
 then
 	VN=$(cat version) || VN="$DEF_VER"
 elif test -d .git -o -f .git &&
-	VN=$(git describe --abbrev=4 HEAD 2>/dev/null) &&
+	VN=$(git describe --match "v[0-9]*" --abbrev=4 HEAD 2>/dev/null) &&
 	case "$VN" in
 	*$LF*) (exit 1) ;;
 	v[0-9]*)