[MERGE] [bug #158333] Fix RemoteRepository.fetch() and add tests.

[John Arbash Meinel]

This fixes bug #158333
It prefers to import the module, and then use revision.is_null() in
various places, rather than the manual (revision_id in (NULL_REVISION,
None)) and other such checks.

It also adds some tests for repository_implementations that makes sure
Repo.fetch(self) does what is expected (it checks that the revision is
present, but nothing else.)

Bug #158333 is a regression from bzr 0.91, so this (or a variant of it)
should probably be merged into 0.92 final.

John
=:->
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