abuild: microoptimization. call git describe only once

Use a global var for last_commit so we dont need call git for every
subpackage

abuild.in

@@ -808,10 +808,11 @@ EOF
 		done
 	fi
 
-	local last_commit="$(git_last_commit)$(git_dirty)"
-	if [ -n "$last_commit" ]; then
-		echo "commit = $last_commit" >> "$pkginfo"
+	# store last_commit in global var so we only call git once
+	if [ -z "$last_commit" ]; then
+		last_commit="$(git_last_commit)$(git_dirty)"
 	fi
+	echo "commit = $last_commit" >> "$pkginfo"
 
 	get_maintainer
 	if [ -n "$maintainer" ]; then