[Contributing] Remove trailing commas from `public` in a pre-commit hook

Starting more ore less simple with a shell script calling `sed`, which
should work anywhere Git is used.

CONTRIBUTING.md

@@ -17,6 +17,15 @@ For starters, simply naming functions or global variables to reflect their
 actual intent will already be helpful. *Any* name is better than
 `sub_<something>`, and can always be fixed or improved later.
 
+## Setup
+
+Before committing anything, enable the Git hooks from the `hooks/` subdirectory
+to automatically catch common mistakes:
+
+```shell
+git config --local core.hooksPath hooks/
+```
+
 # Contribution guidelines
 
 ## Rule #1

hooks/pre-commit

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+if git rev-parse --verify HEAD >/dev/null 2>&1
+then
+	against=HEAD
+else
+	# Initial commit: diff against an empty tree object
+	against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
+fi
+
+# Remove trailing commas from `public` directives. These crash on TASM32
+# version 5.0, but work on later versions.
+for f in `git diff-index --name-only --cached --diff-filter=ACMRX ${against} | grep \.asm$ -i`; do
+	if [ -n "$(sed -i -E 's/(public\s+[^;]+?),\s*(;.*)?$/\1\2/w /dev/stdout' "$f")" ]; then
+		echo "$f"': removed a trailing comma from a `public` line, please re-stage.'
+		exit 1
+	fi
+done