Trivial cleanups.
* bzrlib/tests/test_shelf.py: 
Fix imports.
* bzrlib/tests/test_osutils.py: 
Fix imports.
* bzrlib/tests/test_dirstate.py: 
Fix imports.
* bzrlib/tests/test_bundle.py: 
Fix imports.
* bzrlib/merge_directive.py:
(MergeDirectiveFormatRegistry): Add Aaron explanation for the
double registration.
* bzrlib/hashcache.py:
Fix imports.