Allow diff generation via ArcanistBundle to be limited to an approximate maximum byte size

Summary:
Ref T13137. See PHI592. When you have a diff with 600MB of videos, we want to bail out of diff generation early (as soon as we realize what we're dealing with), not build an 800MB text diff in memory and then throw it away.

Support bailout during diff generation once we realize we're in over our heads.

This is approximate, but since the limit is fairly large (512KB by default) it isn't too important to be precise.

Test Plan: Rigged some callers to set various byte limits, generated diffs including diffs with large binaries. Got an appropriate diff or exeception depending on how low the limit was.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13137

Differential Revision: https://secure.phabricator.com/D19444