git: add hook prepare-commit-msg

.githooks/prepare-commit-msg

@@ -0,0 +1,45 @@
+#!/bin/sh
+#
+# This hook adds prefix "<repo>/<pkgname>: " to the commit message when
+# committing changes of a single package.
+#
+MSG_FILE="$1"
+SOURCE="$2"
+
+longest_common_prefix() {
+	awk -F/ '
+		(NR == 1) { split($0, prefix); prefix_len = NF }
+		(NR > 1) {
+			for (i = 1; i <= prefix_len; i++) {
+				if (prefix[i] != $i) {
+					prefix_len = i - 1; break
+				}
+			}
+		}
+		(prefix_len == 0) { exit }
+		END {
+			res = prefix[1]
+			for (i = 2; i <= prefix_len; i++) {
+				res = res FS prefix[i]
+			}
+			print(res)
+		}'
+}
+
+prepend_msg() {
+	local prefix="$1"
+
+	printf '%s\n%s\n' "$prefix" "$(cat "$MSG_FILE")" > "$MSG_FILE"
+}
+
+
+# Do nothing if message has been given using -m, template, merge etc.
+[ -z "$SOURCE" ] || exit 0
+
+lcp=$(git diff-index --name-only --cached HEAD | longest_common_prefix)
+
+case "$lcp" in
+	[^.]*/*) prepend_msg "$(echo "$lcp" | cut -d/ -f1-2): ";;
+esac
+
+exit 0