Use the same merge strategy as gerrit itself.
Use the same merge strategy as gerrit itself.

Right now, we use a different merge strategy than Gerrit, which means
that we can successfully cherry-pick patches even if Gerrit can't.
This results in a conflict when Gerrit tries to merge changes. Fix that.

BUG=chromium:311334, chromium:309800
TEST=Verify locally that --strategy=resolve solves the problem.
TEST=Unit tests.
TEST=--buildbot --debug run of mario-paladin

Change-Id: Ib280ca360551f964d084887c07f93893b8214727
Reviewed-on: https://chromium-review.googlesource.com/174544
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite