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
7d6601cf02