A small number of merge request diff files have `binary: true` set
inappropriately. During the external diffs migration on GitLab.com,
approximately 30 in every million (0.003%) fail because the `diffs`
column is the plain diff, rather than the base64-encoded diff.

It would be nicer to correct the data, but a quick and easy fix is to
fall back to the raw column data if the diff is unreadable as base64.

This is particularly safe because the data must *also* be processed as
a diff before being shown to the user, and two characters very commonly
seen in diffs - `-` and `@` - are not valid base64 characters.

This change will fix display and migration of these broken diffs, and
as a happy side effect, the migration to external storage, along with
this change, will also fix the `binary` column for us, so the error
will correct over time.