Add --partial_ok to compare_branches.py.
Add --partial_ok to compare_branches.py.

This change lets compare_branches.py succeed with a partial set of
cherry-picks. In our case, we sometimes get baacked up with several
commits needing to be cherry-picked. The first few cherry-pick fine, and
there's a problematic commit sometime down the line. By accepting the
first few that cherry-pick fine, someone resolving the conflict can
start at where that conflict begins, rather than having to wrangle more
commits unnecessarily.

I tested this with and without the flag, and confirmed that if the
first commit is problematic for cherry-picks, the command does
fail.

Change-Id: I2a8b34577f9cb74565adf90a2b7d5328bc555f85
Reviewed-on: http://gerrit.cloudera.org:8080/10025
Reviewed-by: Joe McDonnell <2f04924382c6a21f96a9a4f8cbedc35ff4599584@cloudera.com>
Tested-by: Philip Zeyliger <3ba3974e96044dcde818e6310ed799abe10671d4@cloudera.com>

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala