R1:de4d7e1b1064

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.