This ensures the issues/MR cache of the sidebar is only updated when the
state or confidential flags changes, instead of changing this for every
update.

Our state machine shall have enough knowledge to not allow states other
than locked from transitioning to opened (e.g. MergeRequest#unlock_mr).

So that whenever we want to reserve more, we're aware,
and don't mess it up.

Every project page displays a navigation menu that in turn displays the
number of open issues and merge requests. This means that for every
project page we run two COUNT(*) queries, each taking up roughly 30
milliseconds on GitLab.com. By caching these numbers and refreshing them
whenever necessary we can reduce loading times of all these pages by up
to roughly 60 milliseconds.

The number of open issues does not include confidential issues. This is
a trade-off to keep the code simple and to ensure refreshing the data
only needs 2 COUNT(*) queries instead of 3. A downside is that if a
project only has 5 confidential issues the counter will be set to 0.

Because we now have 3 similar counting service classes the code
previously used in Projects::ForksCountService has mostly been moved to
Projects::CountService, which in turn is reused by the various service
classes.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36622

So that we could limit the access to Repository#fetch_ref
See: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13416#note_37487433

Also fix a few tests

so that we don't have to fetch it for non-forks

Having two states that essentially mean the same thing is very much like
having a boolean "true" and boolean "mostly-true": it's rather silly.
This commit merges the "reopened" state into the "opened" state while
taking care of system notes still showing messages along the lines of
"Alice reopened this issue".

A big benefit from having only two states (opened and closed) is that
indexing and querying becomes simpler and more performant. For example,
to get all the opened queries we no longer have to query both states:

    SELECT *
    FROM issues
    WHERE project_id = 2
    AND state IN ('opened', 'reopened');

Instead we can query a single state directly, which can be much faster:

    SELECT *
    FROM issues
    WHERE project_id = 2
    AND state = 'opened';

Further, only having two states makes indexing easier as we will only
ever filter (and thus scan an index) using a single value. Partial
indexes could help but aren't supported on MySQL, complicating the
development process and not being helpful for MySQL.

Identified via `ENABLE_BULLET=1 bundle exec rspec spec/requests/api/merge_requests_spec.rb:34`

Improves speed of #34159

For merge requests created after 9.4, we have a `merge_request_diff_commits`
table we can get all the SHAs from very quickly. We just need to exclude these
when we load from the legacy format, by ignoring diffs with no serialised
commits.

Once these have been migrated in the background, every MR will see this
improvement.

This is an ID-less table with just three columns: an association to the merge
request diff the commit belongs to, the relative order of the commit within the
merge request diff, and the commit SHA itself.

Previously we stored much more information about the commits, so that we could
display them even when they were deleted from the repo. Since 8.0, we ensure
that those commits are kept around for as long as the target repo itself is, so
we don't need to duplicate that data in the database.

This cop has been renamed to ActiveRecordSerialize to match the way
"ActiveRecord" is usually written.

This is allowed for existing instances so we don't end up 76 offenses
right away, but for new code one should _only_ use this if they _have_
to remove non database data. Even then it's usually better to do this in
a service class as this gives you more control over how to remove the
data (e.g. in bulk).

This removes the need for relying on Rails' "dependent" option for data
removal, which is _incredibly_ slow (even when using :delete_all) when
deleting large amounts of data. This also ensures data consistency is
enforced on DB level and not on application level (something Rails is
really bad at).

This commit also includes various migrations to add foreign keys to
tables that eventually point to "projects" to ensure no rows get
orphaned upon removing a project.

Closes #34052

To make it simpler to assign users to an Issuable, make MergeRequest
support the attribute `assignee_ids`.

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/27070

Deprecate "chat commands" in favor of "slash commands"

We looked for things like:

 - `slash commmand`
 - `slash_command`
 - `slash-command`
 - `SlashCommand`

This Cop blacklists the use of ActiveRecord's "serialize" method, except
for cases where we already use this.