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.