Check for extensions.hgsubversion

Review Request #11144 — Created Aug. 20, 2020 and submitted — Latest diff uploaded

Information

RBTools
release-2.0.x
8837550...

Reviewers

Previously rbtools tried to detect hgsubversion even there
is no chance that the check could be successfully.

This check saves a slow call of hg binary.

Added hgsubversion extension and saw that rbtools checked for
"hg svn". Removed that extension and saw no check at all.