Meta: Add a post-commit commit message linter hook

This should help with getting commit messages tidy before they pass
through CI's commit linter :^)

For this hook to work pre-commit has to be explicitly installed via:
`pre-commit install --hook-type commit-msg`

Meta/lint-commit.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+
+# the file containing the commit message is passed as the first argument
+commit_file="$1"
+commit_message=$(cat "$commit_file")
+
+error() {
+  echo -e "\033[0;31m$1:\033[0m"
+  echo "$commit_message"
+  exit 1
+}
+
+# fail if the commit message contains windows style line breaks (carriage returns)
+if grep -q -U $'\x0D' "$commit_file"; then
+  error "Commit message contains CRLF line breaks (only unix-style LF linebreaks are allowed)"
+fi
+
+line_number=0
+while read -r line; do
+  # ignore comment lines
+  [[ "$line" =~ ^#.* ]] && continue
+
+  ((line_number += 1))
+  line_length=${#line}
+
+  category_pattern="^\S.*?: .+"
+  if [[ $line_number -eq 1 ]] && (echo "$line" | grep -P -v -q "$category_pattern"); then
+    error "Missing category in commit title (if this is a fix up of a previous commit, it should be squashed)"
+  fi
+
+  if [[ $line_length -gt 72 ]]; then
+    error "Commit message lines are too long (maximum allowed is 72 characters)"
+  fi
+done <"$commit_file"
+exit 0