rbt patch could historically only fetch published diffs, and not draft

diffs. This meant that a user could not re-download an in-progress patch

from a review request, and automated build systems could not test or

inspect diffs prior to publishing.
This change adds support for fetching draft diffs, when available. There

are two conditions in which we're fetching a draft diff:


We're fetching the latest diff (default). If found, a draft diff will

   take precedence over any published diffs (as they are the latest),

   and will avoid looking up diff revisions.


We're fetching a specific diff revision (using --diff-revision),

   and the diff revision in question is not published. We'll fetch the

   draft, and if the revision matches, it'll be returned.


Only the owner of a review request or a special user with the

can_edit_review_request permission can fetch draft diffs. For other

users, only published diffs are considered, as normal (this is managed

by the Review Board API and not RBTools).

Tested patching/printing the following:

Latest published diffs
Latest draft diff
Specific diff revision of published diff
Specific diff revision of draft diff
Invalid diff revision

Tested these as the owner, as a user with the special permission, and

as a standard user, ensuring the correct behavior.