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.

Reviewers: Makinde, aran, jungejason, tuomaspelkonen

Reviewed By: Makinde

CC: aran, Makinde

Differential Revision: 793