This is already defined in `ExtractsPath`, which this controller
includes.

This reverts merge request !15712

- Apply some refactoring for code reuse
- Add file status validation for Files::DeleteService
- Write additional specs

When commiting multiple files we're now checking if any of those files
has been modified by another commit and we're rejecting the new commit
in this case.

Previously, this would include the entire User record in the update
hash, which was rendered in the response using `to_json`, erroneously
exposing every attribute of that record, including their (now removed)
private token.

Now we only include the user ID, and perform the lookup on-demand.

If Unlinking a fork would fail somewhere after this, the LFS objects
might still be linked. Which would cause issues when trying to destroy
a project.

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.