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

Introduce default_git_depth in project's CI/CD settings and set it to
50. Use it if there is no GIT_DEPTH variable specified. Apply this
default only to newly created projects and keep it nil for old ones
in order to not break pipelines that rely on non-shallow clones.

default_git_depth can be updated from CI/CD Settings in the UI, must be
either nil or integer between 0 and 1000 (incl).

Inherit default_git_depth from the origin project when forking projects.

MR pipelines are run on a MR ref (refs/merge-requests/:iid/merge) and it
contains unique commit (i.e. merge commit) which doesn't exist in the
other branch/tags refs. We need to add it cause otherwise it may break
pipelines for old projects that have already enabled Pipelines for merge
results and have git depth 0.

Document new default_git_depth project CI/CD setting

Add web_url to tree entry in GraphQL API

See merge request gitlab-org/gitlab-ce!28646

Revert a default GIT_DEPTH for MR pipeline

See merge request gitlab-org/gitlab-ce!28926

Refactor and abstract Auto Merge Processes

See merge request gitlab-org/gitlab-ce!28595

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec

and allow passing of child_complexity to the
'resolver_complexity' metho