Check for hg executable only if needed

Review Request #9708 — Created Feb. 28, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
a11323f...

Reviewers

If a Review Board instance uses hgweb only
it is not necessary to have a hg exeutable
on that system.

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/