Fix 'interdiff-revision' field name in the diff context resource.

When I added this resource, I copied the field names that are used for POST/PUT
operations (which use underscores), rather than those in GET operations (which
use hyphens). This worked from the web UI (where we assemble the URL directly),
but caused problems when using the RBTools API (which transforms all
underscores into hyphens for GET requests). This change fixes that field name
to be 'interdiff-revision'.

Testing done:
Verified that diff and interdiff selection still worked properly using the new
URL parameters.

Reviewed at https://reviews.reviewboard.org/r/6292/