config/git: add generic prepare-commit-msg hook

.config/git/prepare-commit-msg

@@ -0,0 +1,34 @@
+#!/bin/sh -e
+#
+# This hook adds a comment for guidance about the commit message
+# format on top of the default commit message.
+#
+# Called by "git commit" with the name of the file that has the
+# commit message, followed by the description of the commit
+# message's source.	The hook's purpose is to edit the commit
+# message file.	If the hook fails with a non-zero status,
+# the commit is aborted.
+#
+# To enable this hook, set the hooksPath in git:
+# git config core.hooksPath .dev/githooks
+
+COMMIT_MSG_FILE=$1
+COMMIT_SOURCE=$2
+SHA1=$3
+
+beginswith() { case $2 in "$1"*) true;; *) false;; esac; }
+
+original=$(cat "$COMMIT_MSG_FILE")
+if beginswith $'\n#' "$original"; then
+	# Find common path prefix of changed files
+	path=$(while read file
+		do test -z "$count" && common="$file" && count=$(expr length "$file") ||
+			while expr substr "$file" $count 1 != substr "$common" $count 1 >/dev/null; do let count--; done
+		done <<<"$(git -P diff --cached --name-only -r)" &&
+			expr substr "${common%.}" 1 "$count")
+	case "$path" in ([0-9]*) path="${path#*/}";; esac
+	{
+      echo "${path%/}:"
+		echo "$original"
+	} > "$COMMIT_MSG_FILE"
+fi