Use bundle.mainreporoot as "hg root".
Use bundle.mainreporoot as "hg root".

The current repository root is available in the active hg config as
`bundle.mainreporoot`. Fetching it from the already-loaded hgrc contents
allows us to skip one more expensive invocation of `hg`.

Testing Done:
- Ran unit tests.
- Checked the debug output of rbt post to verify that it did not call hg root,
  and uses the correct path.

Reviewed at https://reviews.reviewboard.org/r/11146/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools