R1:ee32c186dd82

Stop computing ownership for changed paths for Very Large revisions

Summary:
Depends on D19416. Ref T13110. Ref T13130. See PHI598. When rendering a "Very Large" revision (affecting more than 1,000 files) we currently compute the package/changeset ownership map normally.

This is basically a big list of which packages own which of the files affected by the change. We use it to:

# Show which packages own each file in the table of contents.
# Show an "(Owns No Changed Paths)" hint in the reviewers list…