Don't let blame run for longer than 15 seconds

Summary: Fixes T2450. If we spend more than 15 seconds in blame, just cut it off.

Test Plan:

• Changed timeout to 0.01 seconds.
• Did blame on a non-highlighted file, got no blame, saw warning.
• Did blame on a highlighted file, got no blame.
  • Note: you don't get a warning here because of Ajax stuff. It'd be kind of tricky to add and doesn't seem like a big deal so I'm planning to leave it as-is for now.

Reviewers: chad

Reviewed By: chad

Subscribers: 20after4, chasemp

Maniphest Tasks: T2450

Differential Revision: https://secure.phabricator.com/D14964