Refs T13546
When using `arc land` in a mercurial repository (with `history.immutable = false`) the resulting working state is not currently expected:
1. The `--onto` bookmark will not have advanced to the newly pushed commit(s)
2. The working directory will typically be the old `--onto` changeset
This change updates the behavior which should usually be generally expected:
1. Advance the `--onto` bookmarks, which could be accomplished by `hg pull -B [bookmark1] -B [bookmark2] ...` after landing (though there's a window of opportunity that this would pull additional changes down, but this would likely be fine in most cases?)
2. During the merge/rebase, if the pre-workflow local state's active commit is merged/rebased then track that commit for use in `reconcileLocalState()`. During `reconcileLocalState()` update the working directory to the new commit (or onto-marker bookmark if applicable) as long as there was only a single onto-marker destination. If the working directory is landed onto multiple destinations then the desired updated state is ambiguous.