Improve "arc land" behavior in the presence of merge conflicts and change sequences
Ref T13546. When we encounter a merge conflict, suggest "--incremental" if it's likely to help.
When merging multiple changes, rebase ranges before merging them. This reduces conflicts when landing sequences of changes.
Test Plan: Ran "arc land" to land multiple changes. Hit better merge conflict messaging, then survived merge conflicts entirely.
Maniphest Tasks: T13546
Differential Revision: https://secure.phabricator.com/D21339