Only recognize "Fixes ..." in main revision content like the Summary or Test Plan

Summary:
Fixes T12642. Currently, writing "Fixes T..." in a comment gets picked up as a formal "fixes".

This is a bit confusing, and can also give you a "no effect" error if you "fixes ..." a task which is already "fixes"'d.

We could make the duplicate action a non-error, but just prevent the text from having an effect instead, which seems cleaner.

Test Plan:

• Wrote "Fixes ..." in a summary, saw a "fixes" relationship established.
• Wrote "Fixes ..." in a comment, got a "mention" instead.
• var_dump()'d some stuff as a sanity check, looked reasonable.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12642

Differential Revision: https://secure.phabricator.com/D17805