Have core read environment info from local clone of repo

[sb10]

The initial design has core read environment information from the gitlab repo using a remote call.

There was a discussion that concluded that core should be the only thing writing to the repo, to avoid any conflict possibilities.

That being the case, I think it makes sense to keep the change Sam and I made recently that has core read environment info from the local clone, not direct from the remote. This not only helps with testing, but also gives us a probably-good-enough caching system for free (the OS will likely cache all the disk reads we do), rather than have to make a remote call to gitlab on every front-end search request.

[sb10]

(Sam did ask about this change before we implanted it and you agreed in principle: it's the change from self.repo.lookup_reference(self.reference).peel().tree[path] to self.repo.head.peel(pygit2.Tree)[path], which are functionally equivalent assuming only one core service modifies the repo.)