This reverts commit a82a5957, reversing
changes made to e7df959b.

Enables frozens string for the following:

* lib/gitlab/conflict/**/*.rb
* lib/gitlab/cross_project_access/**/*.rb
* lib/gitlab/cycle_analytics/**/*.rb
* lib/gitlab/data_builder/**/*.rb
* lib/gitlab/database/**/*.rb
* lib/gitlab/dependency_linker/**/*.rb
* lib/gitlab/diff/**/*.rb
* lib/gitlab/downtime_check/**/*.rb
* lib/gitlab/email/**/*.rb
* lib/gitlab/etag_caching/**/*.rb

Partially addresses gitlab-org/gitlab-ce#47424.

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

If a file is deleted, its new_pos is 0 (less than
total_blob_lines), but there is no reason to add the
bottom 'match' line in this case because there is not
extra content which could be expanded.

This currently causes 500's errors when loading the MR page
(Discussion) in a few scenarios.

We were not considering detailed diff headers such as
"--- a/doc/update/mysql_to_postgresql.md\n+++ b/doc/update/mysql_to_postgresql.md"
to crop the diff. In order to address it, we're now using
Gitlab::Diff::Parser, clean the diffs and builds Gitlab::Diff::Line objects
we can iterate and filter on.

We request Gitaly in a N+1 manner to build discussion diffs. Once the diffs are from different revisions, it's hard to make a single request to the service in order to build the whole response.
With this change we solve this problem and simplify a lot fetching this piece of info.

These can be a `BatchLoader` which is proxying a nil, while not being concrete
nils themselves.

Old merge requests can have diffs without corresponding blobs. (This also may be
possible for commit diffs in corrupt repositories.)

We can't use the `&.` operator on the blobs, because the blob objects are never
nil, but `BatchLoader` instances that delegate to `Blob`. We can't use
`Object#try`, because `Blob` doesn't inherit from `Object`.

`BatchLoader` provides a `__sync` method that returns the delegated object, but
using `itself` also works because it's forwarded, and will work for
non-`BatchLoader` instances too. So the simplest solution is to just use that
with the `&.` operator.

Improve performance of DiffDiscussion#truncated_diff_lines and DiffNote#diff_line by removing expensive diff position calculation and comparison

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

Having a distinct class just for that was a bit overkill

Rename classes to (hopefully) clearer names while we're doing that.

Closes gitaly#524

For some old merge requests, we don't have enough information to figure out the
old blob and the new blob for the file. This means that we can't highlight the
diff correctly, but we can still display it without highlighting.

We didn't have a fallback case before, because we believed the conditions were
exhaustive. They weren't, so we can always fallback to not previewing.

* Added keyword arguments to truncated_diff_lines method to allow for using highlighting or not (html templates vs. text)
* Tweaked templates for consistency and format appropriateness

Previously the `truncated_diff_lines` method for outputting a discussion diff took in already highlighted lines, which meant it wasn't reuseable for truncating ANY lines. In the way it was used, it also meant that for any email truncation, the whole diff was being highlighted before being truncated, meaning wasted time highlighting lines that wouldn't even be used (granted, they were being memoized, so perhaps this wasn't that great of an issue). I refactored truncation away from highlighting, in order to truncate formatted diffs for text templates in email, using `>`s to designate each line, but otherwise retaining the parsing already done to create `diff_lines`.

Additionally, while notes on merge requests or commits had already been tested, there was no existing test for notes on a diff on an MR or commit. Added mailer tests for such, and a unit test for truncating diff lines.

Steps to reproduce:

1. Start with a repo with an image
2. Add a commit to delete the image
3. Add another commit to replace the image with another image

In a diff comparison, we really just compare about what the image was before the diff, not
the direct parent of the last commit. This MR fixes that.

Closes #3893, gitlab-org/gitlab-ee#678
Signed-off-by: NRémy Coutable <remy@rymai.me>

In this case comparing old_path and new_path is not enough because there
are two entires that match.

In this specific case using file_path as a cache key is not enough,
because there are two entries with the same path.
Closes #21610.

Instead calling diff_collection.count use diff_collection.size which is cache on the diff_collection

Introducing the concept of SafeDiffs which relates 
diffs with UI highlighting.