doc: Add commit-msg hook

CONTRIBUTING.md

@@ -59,9 +59,12 @@ Git commits should follow the seven rules in [How to Write a Git Commit
 Message][commit]. Also, prefix each commit message with the package it
 affects, e.g. `proto: Use smaller buffer for partial downloads`. If a commit
 affects multiple packages, use your best judgment to pick the most important
-one, or use `all`. Lastly, put any references to issues (e.g. `Fixes #1234`)
-in the issue body, **not** in the commit message. This prevents an issue from
-being referenced over and over by the same commit when amending/rebasing.
+one, or use `all`. A commit-msg hook is provided to enforce this style; to
+install it, run `ln -s ../../commit-msg.sh .git/hooks/pre-commit`.
+
+Lastly, put any references to issues (e.g. `Fixes #1234`) in the issue body,
+**not** in the commit message. This prevents an issue from being referenced
+over and over by the same commit when amending/rebasing.
 
 Speaking of which, don't be afraid to rebase heavily when modifying a PR in
 response to review comments. Avoid commits with messages like `Address review

commit-msg.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+#
+# A commit-msg hook to enforce the us commit message style.
+
+if ! egrep ': [A-Z0-9]+' "$1" >/dev/null; then
+	echo >&2 "Commit message should be of the form \"lowercase: Uppercase\""
+	echo >&2 "(Your commit message was saved in $1)"
+	exit 1
+fi
+if head -n1 "$1" | grep "\.$"; then
+	echo >&2 "Commit message should not end in a period"
+	echo >&2 "(Your commit message was saved in $1)"
+	exit 1
+fi