framework: add hook to detect whether new ports are hooked into the tree

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.

Example run output:
> mkdir -p graphics/test_port
> touch graphics/test_port/Makefile
> git add graphics/test_port/Makefile
> git commit -m "graphics/test_port: not hooked into the build"
[pre-commit] ERROR: Missing 'SUBDIR += test_port' in graphics/Makefile

This should hopefully help reduce the number of index breakages.

Differential Revision: https://reviews.freebsd.org/D34015