The change that improved parent base computation had an unintended side

effect of breaking use of the tracking branch. In the case where the

current working branch was local-only, it seemed to work, but if the

working branch was pushed to remote, it would break saying that there

were no diffs, regardless of the configured tracking branch.
This change fixes that up so that when no revisions are passed in on the

command line, we use the configured tracking/parent branch as the parent

revision, and use the fancy new youngest-ancestor procedure to determine

the merge base of the parent. This gives us the desired behavior:

posting the changes users want, but avoiding (when possible) huge parent

diffs.
A few of the unit tests needed to be fixed to be more correct to match

the desired behavior.

Posted some various changes.
Ran unit tests.