R1:4da43b31a307

Add Mercurial repository configuration and local pull support

Summary: No actual parsing/import yet, but now you can define and pull Mercurial
repositories. I merged most of the local pull code so we can share it between
hg/git.

Test Plan:
- Created a new Mercurial repository to track Codeigniter off Bitbucket
- Edited / saved / etc.
- Launched the mercurial pull daemon, it pulled the repo. Killed and
relaunched, it updated the repo.
- Launched the git fetch deamon, it still works correctly.