Don't check for hg executable if we're using hgweb.
Don't check for hg executable if we're using hgweb.

If a Review Board instance uses hgweb for all communication with Mercurial
repositories, it is not necessary to have a hg executable on that system.

Testing Done:
Removed hg executable from system and saw that a diff can be shown und a new
review request could be opened by post-commit /r/new/

Reviewed at https://reviews.reviewboard.org/r/9708/

chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard