repopick: fix broken pipe error from "repo list"

  * the subprocess running "repo list" was not closed cleanly
    if the branch was found

Change-Id: Ie659966f3a670b25ab076a8dc723c4235760d04e
repopick: skip a cherry pick if its already been merged

* override this behavior with the -f argument

Change-Id: I280b4945344e7483a192d1c9769c42b8fd2a2aef