Support arbitrarily long filenames in Differential

Summary:
Fixes T11660. Currently, if you try to diff a path with more than 255 total characters, we fail to create the diff because we have a text255 column.

There are actually two issues here:

• File names may be arbitrarily long (T11660).
• File names may not be UTF8 (T6633, etc). This is much more complicated and has other issues -- largely that we can't JSON-encode non-UTF8 filenames. I'm punting on that for now and will deal with it later. This doesn't specifically address non-UTF8 paths, although it is a change that's (probably?) required to eventually support them.

This will cause some potentially slow migrations, but better to do them now, if possible, so we have fewer complicated/slow upgrades overall.

Test Plan:
Created a change touching file: very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_directory_name/very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_long_filename.txt

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11660

Differential Revision: https://secure.phabricator.com/D17062