repopick: Use upstream instead of revision to determine branch

This is to allow repopick to determine a branch even on a
revision locked manifest. If upstream is not set, it falls
back to revision.

Per the repo manifest documentation:
Attribute upstream: Name of the Git ref in which a sha1 can be
found. Used when syncing a revision locked manifest in -c mode to
avoid having to sync the entire ref space. Project elements not
setting their own upstream will inherit this value.

Change-Id: I12876f7e3b440f9eab6d1b96eba9b18a13cff2e0