evolve: make `--continue` continue evolving all remaining revisions This patch makes `hg evolve --continue` to continue all remaining revisions instead of just the current conflicted one. There are some still improvements required to this behavior after which the CHANGELOG entry will be added.

tests: add more tests to test-evolve-continue.t This patch adds more tests for `hg evolve --continue`. The tests demonstrates how the continuation handling of hg evolve is broken and the later patches fixes that handling.

evolve: handle case when newnode can be None while continuing evolve There can be case when `hg evolve --continue` is run, when working directory is clean and there is nothing to be included in the new commit. Let's make sure we handle such cases correctly and obsolete the node which is evolved with parent of wdir as successor. This is same as what rebase does.