Don't let users write summaries or test plans which will become ambiguous in commit messages

Summary:
Ref T11085. To recreate the issue:

• From the web UI, click "Edit Revision".
• Write something like this as your "Summary" (i.e., put another field marker, like "Test Plan:", into the summary):

This is a test of the
Test Plan: field to see
if it works.

• Save changes.

Later, when the summary is amended into a commit message, the parser will see two "Test Plan:" fields and fail to parse the message.

Instead, prevent users from making this edit.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11085

Differential Revision: https://secure.phabricator.com/D16846