git-version-gen: Avoid further processing when tarball-version is present

In case a tarball-version file is present, use that and quit.

Otherwise git will continue looking for directories, potentially
finding .git directories which are dirty and mark the version as such.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=90936

git-version-gen

@@ -84,7 +84,10 @@ then
     v=`cat $tarball_version_file` || exit 1
     case $v in
 	*$nl*) v= ;; # reject multi-line output
-	[0-9]*) ;;
+	[0-9]*)
+		echo "$v" | tr -d '\012'
+		exit 0
+		;;
 	*) v= ;;
     esac
     test -z "$v" \