Truncate the one-line diff update summary when updating a revision to 250 bytes

Summary:
Fixes T7899. If you create or update a revision and type an enormously long first line, we currently fatal trying to insert it into the database.

This text is only used to show a single-line summary of the diff in the "History" tab, which should probably be updated anyway. For now, stop fataling.

Test Plan:
Uploaded a diff with the description "MMMM..." (thousands of them).

Before patch: fatal on description being too long.
After patch: beautiful "MMMM" summary.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7899

Differential Revision: https://secure.phabricator.com/D17038