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.

Now, when requesting a commit from the Repository model, the results are
not cached. This means we're fetching the same commit by oid multiple times
during the same request. To prevent us from doing this, we now cache
results. Caching is done only based on object id (aka SHA).

Given we cache on the Repository model, results are scoped to the
associated project, eventhough the change of two repositories having the
same oids for different commits is small.

Previous efforts were aimed at detecting N + 1 queries, general
regressions are hard to find and mitigate.

The pipeline was quite meagre in both stages and the number of groups.
This has been improved. Performance is not yet optimal, but to limit
this from sliding further this slippery slope, a hard limit has been
set.

Removed all the unnecessary include of `WaitForAjax` and `ApiHelpers` in the specs.
Removed unnecessary usage of `api:true`

Use detailed_status effectively. Remove unnecesarry context(nest). Add new context in merge_requests_controller_spec.rb and fix a bug. Correct description of spec.

Fix broken test

[ci skip]