Fix --parent situation for mercurial.
The mercurial implementation was using --parent to specify the upstream branch,

when it really ought to have been using --tracking-branch. This made it very

hard to support using --parent to specify an intermediate branch between the

upstream and the tip when posting without explicit revisions.
This change fixes the MercurialClient to use --tracking-branch to override

the detected upstream branch, and then allows --parent to actually generate

parent diffs.

Ran unit tests.
Posted a diff using --parent to my local devserver.