$ git --exec-path
$ ls $(git --exec-path) | head
git will actually pick up anything in $PATH of the form 'git-<command>' and allow you to run it as 'git <command>'. The downside of this, is that commands of the form:
git <command> --help
Edit: Another downside, is that some 'smart' command-line completion settings either have a hard-coded list of 'git <command>' commands, or look for said commands in the value returned by 'git --exec-path'. So you can't tab-complete git-<command> commands that you just drop into $PATH (at least not out of the box).