Stop prepending /usr/bin to hooks PATH, or document it very clearly

On hooks Git automatically prepends to the `PATH` via the `setup_path` function:

- `git --exec-path` (`/usr/lib/git-core`)
- the directory of `$0` (`/usr/bin`) if you call it with the full path

The problem is that the `/usr/bin` breaks "interpreter version manager systems" like RVM, rbenv, virtualenv, etc. since people will write hooks like:

    #!/usr/bin/env ruby

and the `/usr/bin` ruby will get run instead of the managed one (`~/.rvm/some/path/bin).

I recommend either:

- not adding the `$0` if possible (ideal but backwards incompatible
unless a new config is added)
- documenting this behavior *very* clearly on `man githooks`
- proposing some other mechanism that neatly solves the problem

I will try to submit a patch for the desired solution if within my technical capability.

Samples of problems this has caused people:

- https://github.com/gitlabhq/gitlabhq/issues/8045
- http://stackoverflow.com/questions/9037284/how-can-i-run-a-virtualenv-python-script-as-a-git-pre-commit-hook
- http://stackoverflow.com/questions/17515769/why-is-my-ruby-git-script-hook-run-with-the-wrong-path
- https://github.com/sstephenson/rbenv/issues/374
- https://github.com/magit/magit/issues/498