Fix a transaction editor "continue;" inside "switch()" for PHP 7.3

Summary: See https://discourse.phabricator-community.org/t/new-git-commit-processing-fails-on-php-7-3/. This "continue" should be a "break".

Test Plan:

• Tried to assign a task to myself while I was already the owner, got an appropriate error.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19844