Improve diff lines count efficiency

Prepares us for upgrade to Rails 5.2

The `#reload` makes to load all objects into memory,
and the main purpose of `#reload` is to drop the association cache.

The `#reset` seems to solve exactly that case.

Since external diffs are likely to be a bit slower than in-database
ones, add a mode that makes diffs external after they've been obsoleted
by events. This should strike a balance between performance and disk
space.

A background cron drives the majority of migrations, since diffs become
outdated through user actions.

If a diff happened to include a single null byte anywhere, insertion
into the database would fail with an Error 500 since the column is text
and not a byte array. To fix this, we mark the diff as binary if we
detect a single null byte and Base64-encode it.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57710

When a branch is deleted, the push commit IDs is an empty array.
The previous change would attempt to call `[].exists?`, which is
invalid. Fix this by returning `MergeRequestDiffCommit.none` instead.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53853

Fetch database columns directly if available

Previously for a given merge request, we would:

1. Create the array of commit SHAs involved in the push (A)
2. Request all merge request commits and map the SHA (B)
3. Reload the diff if there were any common commits between A and B

We can avoid additional database querying and overhead by
checking with the database whether the merge request contains any
of the commit SHAs.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/53213

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

Partially addresses #47424.

* processes any pending records which are not migrated yet
* bumps import_export version because of new commits_count attribute
* removes commits_count fallback method

This reverts commit 0b7d1085.