Configure now detects VCS info. Configure now detects Watchman.
We can combine the two so configure can detect if Mercurial
is configured with Watchman enabled.

This commit does two things:

1) collects the Mercurial config so it is available to downstream checks
2) examines the config for presence and state of the fsmonitor
   extension

We don't yet do anything with the fsmonitor state. But it should be
useful soon. Also, the return value is kinda wonky. This will almost
certainly be improved as soon as there is an actual consumer.

MozReview-Commit-ID: HyHZ2X8VI0h

--HG--
extra : rebase_source : e53928127470340275f0c0f07db72b536bba885b
extra : source : a8373914cbfd9b8595fc24f36c876cab0a26c02a