repopick: add -b shortcut option

  * useful for people like me who want to pick into temp branch so
    that the picks survive a repo sync
  * shortcut to "--start-branch auto --abandon-first --ignore-missing"

Change-Id: Ia4d4309d27e46a15ff4b74525668d974f4251dcb
repopick: handle additional error cases around subprocesses

  * gracefully error if the project path cannot be found
  * poll() was not a reliable determination of EOF on my
    Darwin-Python 2.7 system.

Change-Id: I203c2a75820f8acc079a5c9751d1c04daf6f3a16