[MERGE][#144421] Fix `bzr st -rbranch:PATH_TO_BRANCH`

[Ian Clatworthy]

Lukáš Lalinský wrote:
> * Add RevisionSpec.as_tree which returns a tree representation of the
> revision specifier. This might return a remote revision tree.
> * Make status use this new method and not require
> RevisionSpec.as_revision_id, which for RevisionSpec_branch fetches data
> to the local repository.
> * Refactor common code for getting either a revision tree or a
> tree/branch's basis tree into a private function _get_revision_tree and
> make it use RevisionSpec.as_tree.

bb:tweak

Over and above Aaron's feedback, the docstring for test_status_write_lock
needs a grammar fix. I'll tweak and merge.