This pre-commit hook looks for newly added files matching
foo/bar/Makefile
For these files, it is checking that a SUBDIR+=bar entry is
present in foo/Makefile.

This is likely not yet all-there, and should/could probably be
extended to also make sure that foo/Makefile is staged too, and
also handle port removals...

Example run output:

	> mkdir -p graphics/test_port
	> touch graphics/test_port/Makefile
	> git add graphics/test_port/Makefile
	> git commit -m "[new port] graphics/test_port: antoine will send you a mail"
	[pre-commit] ERROR: Missing 'SUBDIR += test_port' in graphics/Makefile