PRs with dependents and conflicts across multiple commits are broken when creating downstream syncs

[jgraham]

If we have a PR with several commits C1, C2, C3 and dependent commit D, with merge conflicts in C2 and C3, the following happens:

• We apply C1 in plain_apply.
• We try to apply C2 but hit a conflict.
• We do rebase_apply which rebases onto autoland, but doesn't fix the conflict
• We do dependent_apply which resets to autoland, applies D, applies C1 and fails, leaving the patch file C2 in the working directory.
• C2 is applied manually and the downstream sync is restarted
• We attempt to apply C3 in plain_apply but this fails
• In rebase_apply, we notice that we have a dependent, reset to autoland and try to reapply the commits starting from C1, which again fails.

We could make rebase_apply not reset this case, and just try to apply the dependent, assuming git will handle it correctly if the dependent was already applied due to a landing.