Add project level git depth setting

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

1 files changed, 16 insertions, 0 deletions

diff --git a/doc/user/project/pipelines/settings.md b/doc/user/project/pipelines/settings.md
index 8b762307ac4..16f48c462eb 100644
--- a/doc/user/project/pipelines/settings.md
+++ b/doc/user/project/pipelines/settings.md
@@ -20,6 +20,22 @@ There are two options. Using:
 The default Git strategy can be overridden by the [GIT_STRATEGY variable](../../../ci/yaml/README.md#git-strategy)
 in `.gitlab-ci.yml`.
 
+## Git shallow clone
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28919) in GitLab 12.0.
+
+NOTE: **Note**: As of GitLab 12.0, newly created projects will automaticallyl have a default 
+`git depth` value of `50`.
+
+It is possible to limit the number of changes that GitLab CI/CD will fetch when cloning 
+a repository. Setting a limit to `git depth` can speed up Pipelines execution. Maximum
+allowed value is `1000`.
+
+To disable shallow clone and make GitLab CI/CD fetch all branches and tags each time,
+keep the value empty or set to `0`.
+
+This value can also be [overridden by `GIT_DEPTH`](../../../ci/large_repositories/index.md#shallow-cloning) variable in `.gitlab-ci.yml` file.
+
 ## Timeout
 
 Timeout defines the maximum amount of time in minutes that a job is able run.