Blob.lazy adds a blob to a batch to load at a later point, using the
BatchLoader library. Whenever any lazy blob's attributes are accessed,
all lazy blobs requested to that point will be loaded.

BatchLoader, the library we use for this, should only request items in a
batch once. That is, if we have these batches:

1. a, b, c
2. d, e, f

Then a, b, and c should only be requested in the first batch. But if you
modify the list of items in the batch, then the second batch will
request a, b, c, d, e, f, which is almost certainly not what we want!

https://github.com/exAspArk/batch-loader/issues/44 is the upstream issue
for this, but we can also solve this in our application by not modifying
the arguments we're using inside a BatchLoader batch.

Adds `# frozen_string_literal: true` to spec/models ruby files

This commit, introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23812,
fixes a problem creating a displaying image diff notes when the image
is stored in LFS. The main problem was that `Gitlab::Diff::File` was
returning an invalid valid in `text?` for this kind of files.

It also fixes a rendering problem with other LFS files, like text
ones. They LFS pointer shouldn't be shown when LFS is enabled
for the project, but they were.

By specifying `key`, we get a different lazy batch loader for each
repository, which means that accessing a lazy object from one repository
will only result in that repository's objects being fetched, not those
of other repositories, saving us some unnecessary Gitaly lookups.

After installing a new gem, batch-loader, a construct can be used to
queue data to be fetched in bulk. The gem was also introduced in both
gitlab-org/gitlab-ce!14680 and gitlab-org/gitlab-ce!14846, but those mrs
are not merged yet.

For the generation of diffs, both the old blob and the new blob need to
be loaded. This for every file in the diff, too. Now we collect all
these so we do 1 fetch. Three `.allow_n_plus_1_calls` have been removed,
which I expect to be valid, but this needs to be confirmed by a full CI
run.

Possibly closes:
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37445
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37599
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37431