Fix bug #423331 by adding a way to configure the path used to
store the revid.

* tests/test_upload.py:
(TestUploadMixin.test_upload_revid_path_in_dir): Smoke test that
we can force a different path.
(TestFullUpload.test_full_upload_empty_tree,
TestIncrementalUpload.test_upload_for_the_first_time_do_a_full_upload):
Update references to the revid path.

* __init__.py:
(get_upload_revid_location, set_upload_revid_location): New config
variable.
(BzrUploader.set_uploaded_revid, BzrUploader.get_uploaded_revid):
Use the config variable defining the path.

* README: 
Document the .bzr-upload.revid file and upload_revid_location
configuration variable.