Expose default_git_depth via project API

Closes #62908

See merge request gitlab-org/gitlab-ce!29353

Enable Get and Update of ci_default_git_depth for
Project API.

Renaming Project#default_git_depth to :ci_default_git_depth
to give more context through the API usage.

Add API documentation

Add spec for task_completion_status

Add test cases for task_completion_status result

Extracted shared samples

Add new spec file for task completion status response

Fix style errors

Add changelog entry

Changed samples to Hashes

Remove test for successful request

Remove not nil expectation

Add task_completion_status to api documentation for issues

Add task_completion_status to api documentation for merge_requests

Refactor spec so it just requests one specific item

Add task_completion_status to Taskable

Simplified task completion status in entities

Refactor spec so it separates status code check and content check

Fix spec description text and field name

Currently, the field used to fill the contents is called "code".
Also "visibility" is now required.

Automatically update MR merge-ref along merge status

Closes #58495

See merge request gitlab-org/gitlab-ce!28513

We can query namespaces, and nested projects.

Projects now exposes statistics

This couples the code that transitions the `MergeRequest#merge_status`
and refs/merge-requests/:iid/merge ref update.

In general, instead of directly telling `MergeToRefService` to update
the merge ref, we should rely on `MergeabilityCheckService` to keep
both the merge status and merge ref synced. Now, if the merge_status is
`can_be_merged` it means the merge-ref is also updated to the latest.

We've also updated the logic to be more systematic and less user-based.