misc/dashboard/builder: various cleanups

* allow commit watcher to be disabled, useful for small slow builders who will never be the first to notice a commit.
* builders always update their local master working copy before cloning a specific revision.
* refactor hg repo operations into a new type, Repo.

R=adg, shanemhansen, luitvd
CC=golang-dev
https://codereview.appspot.com/7326053