Build: make repopick.py executable

Change-Id: Ide52d3819d8dac37f8e300f96f8efcbbf993b866
(cherry picked from commit 9a75f2817586610c258321e6e25cc7392b94681f)
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
(cherry picked from commit 30d53f39b0a651c77add72e969f43ba8cdbd6a6f)
repopick: skip a cherry pick if its already been merged

* override this behavior with the -f argument

Change-Id: I280b4945344e7483a192d1c9769c42b8fd2a2aef
(cherry picked from commit f2b12d3c6fd1810058bfd5bbbf1fa24d67dd6910)