GIT-VERSION-GEN: squelch unneeded error from "cat version"

Now this is really a corner case, but if you have the git source
tree from somewhere other than the official tarball, you do not
have version file.  And if git-describe does not work for you
(maybe you do not have git yet), we spilled an error message
from "cat version".

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

GIT-VERSION-GEN

@@ -7,8 +7,11 @@ DEF_VER=v1.2.GIT
 # (included in release tarballs), then default
 if VN=$(git-describe --abbrev=4 HEAD 2>/dev/null); then
 	VN=$(echo "$VN" | sed -e 's/-/./g');
-else
+elif test -f version
+then
 	VN=$(cat version) || VN="$DEF_VER"
+else
+	VN="$DEF_VER"
 fi
 
 VN=$(expr "$VN" : v*'\(.*\)')