Fixes T4189. Ref T4151. Allows repositories to have additional custom hooks for operations which can't be expressed with Herald (one such operation is lint).

This adds only local hook directories, since they're easier to use with existing hooks than global directories. I might add global directories eventually.

This doesn't support Mercurial since we have no demand for it and it's more complicated (we lose compatibility and power by just dropping a hooks.d/ somewhere).

• Pulled hosted SVN and Git repos to verify the hook directories generate correctly.
• Added a variety of hooks to the hook directories (echo + pass, fail).
• Pushed commits and verified the hooks fired (output expected info, or failed).
• Verified push log reflected the correct error code ("3", external) and detail ("nope.sh") when rejecting.