Make requirement update proposals more robust.

When proposing requirement update changes ensure that the proper branch
can be checked out if the branch isn't master. To do this checkout
origin/$BRANCH instead of just branch. Additonally, don't git review -d
an existing change as it may have merge conflicts with the target
branch. Instead make changes against the target branch, push them to the
existing change as a new patchset, and ignore failures of git review
when there are no new changes.

This allows changes to the requirements files to happen out of band from
these automated change proposals.

Closes-Bug: 1274251
Change-Id: I7b81b87a90639a66dce5502cded5e37d96abd1f7
Merge "Make requirement update proposals more robust."