sh subprocess replacement doesn't work on Windows

Tried to release a package on Windows with git. Was able to workaround the issue by using pbs but more of a workaround than a real solution.
sh isn't available for Windows. pbs (predecessor to sh) is but is sh or pbs really necessary? subprocess should do the job just fine.

sh is being used in [rezplugins/release_hook/command.py](https://github.com/nerdvegas/rez/blob/master/src/rezplugins/release_hook/command.py#L12) and [rezplugins/release_vcs/git.py](https://github.com/nerdvegas/rez/blob/master/src/rezplugins/release_vcs/git.py#L7)

I would like to replace sh but wanted to check in what you think about the issue?

Start with src/rezplugins/release_hook/command.py and src/rezplugins/release_vcs/git.py, where the issue identifies the sh imports. Check how those release and Git commands are invoked on Windows and compare the behavior with the reported pbs workaround. Done means package release through Git no longer depends on sh and works on Windows.

Written by the indexing model from the issue text.