Closes gitaly#929

A file containing /:\d+:/ in its contents would break the search results if
those contents were part of the results, because we were splitting on colons,
which can't work with untrusted input.

Changing to use the null byte as a separator is much safer.

with StrongMemoize

Closes #41739

When a project uses fast-forward merging strategy user has
to rebase MRs to target branch before it can be merged.
Now user can do rebase in UI by clicking 'Rebase' button
instead of doing rebase locally.

This feature was already present in EE, this is only backport
of the feature to CE. Couple of changes:
* removed rebase license check
* renamed migration (changed timestamp)

Closes #40301

This reverts merge request !15712

Uses `list_commits_by_oid` on the CommitService, to request the needed
commits for pipelines. These commits are needed to display the user that
created the commit and the commit title.

This includes fixes for tests failing that depended on the commit
being `nil`. However, now these are batch loaded, this doesn't happen
anymore and the commits are an instance of BatchLoader.

Allows ordering in GET api/v4/projects/:project_id/repository/contributors
through `order_by` and `sort` params.
The available `order_by` options are: name|email|commits.
The available `sort` options are: asc|desc.

Closes gitaly#808

This has the side effect of making this method rugged call free, which
is the reason I actually changed this.

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

Conflicts used to take a `Repository` and pass that to
`Gitlab::Highlight.highlight`, which would call `#gitattribute` on the
repository. Now they use a `Gitlab::Git::Repository`, which didn't have that
method defined - but defining it on `Gitlab::Git::Repository` does make it
available on `Repository` through `method_missing`, so we can do that and both
cases will work.

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

This adds an optimised way of getting the latest pipeline status for a
list of Commit objects (or just a single one).

Prior to this MR there were two GitHub related importers:

* Github::Import: the main importer used for GitHub projects
* Gitlab::GithubImport: importer that's somewhat confusingly used for
  importing Gitea projects (apparently they have a compatible API)

This MR renames the Gitea importer to Gitlab::LegacyGithubImport and
introduces a new GitHub importer in the Gitlab::GithubImport namespace.
This new GitHub importer uses Sidekiq for importing multiple resources
in parallel, though it also has the ability to import data sequentially
should this be necessary.

The new code is spread across the following directories:

* lib/gitlab/github_import: this directory contains most of the importer
  code such as the classes used for importing resources.
* app/workers/gitlab/github_import: this directory contains the Sidekiq
  workers, most of which simply use the code from the directory above.
* app/workers/concerns/gitlab/github_import: this directory provides a
  few modules that are included in every GitHub importer worker.

== Stages

The import work is divided into separate stages, with each stage
importing a specific set of data. Stages will schedule the work that
needs to be performed, followed by scheduling a job for the
"AdvanceStageWorker" worker. This worker will periodically check if all
work is completed and schedule the next stage if this is the case. If
work is not yet completed this worker will reschedule itself.

Using this approach we don't have to block threads by calling `sleep()`,
as doing so for large projects could block the thread from doing any
work for many hours.

== Retrying Work

Workers will reschedule themselves whenever necessary. For example,
hitting the GitHub API's rate limit will result in jobs rescheduling
themselves. These jobs are not processed until the rate limit has been
reset.

== User Lookups

Part of the importing process involves looking up user details in the
GitHub API so we can map them to GitLab users. The old importer used
an in-memory cache, but this obviously doesn't work when the work is
spread across different threads.

The new importer uses a Redis cache and makes sure we only perform
API/database calls if absolutely necessary.  Frequently used keys are
refreshed, and lookup misses are also cached; removing the need for
performing API/database calls if we know we don't have the data we're
looking for.

== Performance & Models

The new importer in various places uses raw INSERT statements (as
generated by `Gitlab::Database.bulk_insert`) instead of using Rails
models. This allows us to bypass any validations and callbacks,
drastically reducing the number of SQL queries and Gitaly RPC calls
necessary to import projects.

To ensure the code produces valid data the corresponding tests check if
the produced rows are valid according to the model validation rules.

Signed-off-by: Rémy Coutable <remy@rymai.me>