Add hook for testing commit messages

Makefile.common

@@ -414,6 +414,7 @@ git-clean:
 
 git-sanity-check: git-clean
 	test "./Compiler/boot/bootstrap-sources.tar.xz" = "`find . -type f -size +684k | grep -v 3rdParty`"
+	# These are tested by the commit-msg hook
 	for commit in `git rev-list origin/master..HEAD`; do \
 	  test 50 -ge "`git log --format="%s" $$commit~1..$$commit | wc -c`" || (echo "$$commit has a too long commit summary (leave an empty line after the first if it is not part of the summary)"; git log $$commit~1..$$commit; false) && \
 	  test 72 -ge "`git log --format="%b" $$commit~1..$$commit | wc -L`" || (echo "$$commit has too long commit lines (max 72 characters per line)"; git log $$commit~1..$$commit; false) && \