githooks/prepare-commit-msg: add systemd prefixes

Extend the hook to automatically make useful titles for packages in
systemd directories. We had two styles for modifying the systemd package
itself, either "systemd/systemd:" or "systemd:". Pick the shorter one.

Part-of: https://gitlab.postmarketos.org/postmarketos/pmaports/-/merge_requests/6348

.githooks/prepare-commit-msg

@@ -40,6 +40,12 @@ lcp=$(git diff-index --name-only --cached HEAD | longest_common_prefix)
 prefix=""
 
 case "$lcp" in
+	extra-repos/systemd/systemd/*)
+		prefix="systemd"
+		;;
+	extra-repos/systemd/*)
+		prefix="systemd/$(echo "$lcp" | cut -d/ -f3)"
+		;;
 	device/*/*)
 		prefix="$(echo "$lcp" | cut -d/ -f3)"
 		;;