convert: return commit objects for revisions in the revmap
Source revision data that exists in the revmap are ignored when pulling
data from Perforce as we consider them already imported. In case where
the `convertcmd.convert` algorithm requests a commit object for such
a revision we are creating it. This is usually the case for parent of
the first imported revision.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).