exec-path behavior

I'm wondering about the intended behavior of the --exec-path/GIT_EXEC_PATH option - is it supposed to override or add to the built-in path?  It seems to do some of both.

$ git --exec-path=foo rebase git: 'rebase' is not a git-command. See 'git --help'.

but:

$ git --exec-path=foo status # On branch master nothing to commit (working directory clean)

There are plenty of commands in each category, and it's not immediately obvious to me what criteria separate them.  Not a huge problem, since it's easy enough to use --exec-path=foo:<normal-path>, but I was in the process of adding support for exec-path to the bash completion, and it's an important distinction there - and much easier if it adds to the built-in, of course.

Thanks, Jeffrey