Fix the exception when watching the first commit of a mercurial repo

Summary: Most checks were actually in place, but `ExecFuture` throws a `CommandException` which wasn't taken into account.

Test Plan: look at the first command and no longer saw an exception. Also, other commits worked as well.

Reviewers: richardvanvelzen

Reviewed By: richardvanvelzen

CC: krisbuist, Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7730