Add a missing key to the ManiphestTransaction table

Summary:
Caught a bad (8ms) query that didn't have a good key available. Add an
appropriate key.

The query in question is:

SELECT * FROM `maniphest_transaction` WHERE taskID = 262 ORDER BY id ASC;

...which is used in the task detail view.

Test Plan: Ran EXPLAIN on this query before/after upgrading, it now uses the
newly available key.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: aran

Differential Revision: 1037