Make the --diff-filename and --update flags mutually exclusive.
Make the --diff-filename and --update flags mutually exclusive.

When specifying an external diff file, we can't use --update to automatically
guess which review request to update. This change adds a check for that case.

Testing done:
Tried to run with both arguments and saw the error.

Fixes bug 3755.

Reviewed at https://reviews.reviewboard.org/r/7358/

haosdent/rbtools,haosdent/rbtools,beol/rbtools,reviewboard/rbtools,beol/rbtools,davidt/rbtools,haosdent/rbtools,datjwu/rbtools,datjwu/rbtools,beol/rbtools,reviewboard/rbtools,davidt/rbtools,davidt/rbtools,reviewboard/rbtools,datjwu/rbtools