We introduced support for moved files a while back, but copied files

were never properly supported. Copied files with 100% similarity would

flat-out break.
This change introduces support for parsing the copied file info from git

diffs, storing this state in the database, and showing it in the diff

viewer (just like we show moved files). It doesn't require any schema

modifications in the database.

Unit tests pass.
Uploaded a diff file that was broken in the wild. It had 100% similarity

copy and a 88% similarity rename. Before this change, it failed, but after,

it worked.