Use bundle.mainreporoot as "hg root"

Review Request #11146 — Created Aug. 20, 2020 and submitted — Latest diff uploaded

Information

RBTools
release-2.0.x
2f303b7...

Reviewers

This saves one more hg binary call and replaces
"hg root" with the value of "bundle.mainreporoot" of
"hg showconfig" as it is the same.

Checked the debug outpot of rbt post that
it do not call hg root and uses the correct path.