make sure merge prefixes for git are always forward-slash-separated

Summary:
Git doesn't expect backslashes in tree merge prefixes, but on Windows we
were incorrectly using those. That got things very confused when
anything had an import path with slashes in it. Use pathlib to make sure
we always get a posix path for this.

Fixes https://github.com/buildinspace/peru/issues/60.

Test Plan: New unit test that was previously failing on Windows.

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D136