In the case where the local clone has remotes, but none of them are

named "origin", rbtools was failing to correctly determine the most

recent upstream commit. This was the case even when a tracking branch or

parent branch associated with a remote was configured. The cause here

was a couple places where we were hardcoding "origin" instead of

attempting to detect the remote.
This change fixes that by introducing a new mechanism to try to guess

the remote. If a remote tracking branch is specified, we'll prefer that,

failing back to the remote configured for the current branch. If no

direct association can be found, we'll return either "origin" or fall

back on the first remote found, emitting a warning.

Ran unit tests.
Set up a clone with multiple remotes, none of which were named

  "origin". Saw that I could correctly post changes when branch remotes

  and/or TRACKING_BRANCH were set correctly.