Do repo processing in an existing local dir

Keep repos around so that we can do smarter things with them
aroud processing remotes and tracking descriptions.

Changes upstream tracking from mucking with repos behind gerrit to
pushing in changes with the push command, which should allow for zuul
to take action based on receiving new refs from upstream.`

Change-Id: I9e79489b7c724db1320272f75a7da2c2b4c24607
Merge "Do repo processing in an existing local dir"