R1:579941b866f2

PhabricatorRepository: temporarily change the localPath

Summary:
I'm on a host where I have the PhabricatorRepositoryPullLocalDaemons
tracking a remote repo. In my case, these end up as local git repos in
/var/repo/$name.

I'm working on another daemon that is going to automatically make
changes and commit them back upstream. I figured it would be best to do
this in a new local repo. I'll put these in /var/repo-clones/$name.

It's nice to use the exec*() functions in PhabricatorRepository, so the…