Currently, git-drover gives up and cleans up if the cherry-pick fails.
This change allows the user to manually resolve conflicts when using
git-drover.

BUG=404755

Review URL: https://codereview.chromium.org/1397313002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297429 0039d316-1c4b-4281-b951-d872f2087c98