Do not rewrite base_commit_id if not necessary

Review Request #11145 — Created Aug. 20, 2020 and submitted — Latest diff uploaded

Information

RBTools
release-2.0.x
ed15ce8...

Reviewers

If the base_commit_id does not contains any relative
information we do not need to rewrite it.

This saves one another call of hg binary.

Used a base_commit_id with relative character and saw
the rewrite. Used a normal base_commit_id and saw
that it did not rewrite.