Pull only existing authorPHIDs in blame

Summary:
I've hit this error by exhausting memory limit on blaming a big file with lots of unknown authors.
It triggered the error ~1000 times with stack trace containing the whole ~100 kB file.
The memory ran out when it tried to JSON serialize the stack traces for the DarkConsole.

Test Plan: Blamed file with unknown authors.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5730