add auto-cancel for pending pipelines on branch, if they are not HEAD

fix changelog MR reference

add non-HEAD builds finder and add `created` pipelines to scope

add spec for auto-cancel non-HEAD pipelines and refactor create_pipeline_service_spec

more refactoring for spec

adds option for auto-cancel into CI/CD settings

fix spec to new configuration

fix rubocop

fix schema.rb

fix schema.rb

replace Gitlab 9.0 with 9.1 in doc

change wording on pipeline settings

added auto_canceled_by field as identifier of autocancel subject

remove unnecessary index

replace service with retry_lock

replace auto_cancel_pending_pipelines boolean setting with integer (and enum in model)

fix schema.rb

fix schema.rb

remove projekt attribute and clean up spec

clean up spec withcouple of shared examples

added spec for "It does not cancel current pipeline" scenario

add some specs to auto-cancel

add spec for another branch pipelines

1 files changed, 9 insertions, 0 deletions

diff --git a/doc/user/project/pipelines/settings.md b/doc/user/project/pipelines/settings.md
index c398ac2eb25..88246e22391 100644
--- a/doc/user/project/pipelines/settings.md
+++ b/doc/user/project/pipelines/settings.md
@@ -60,6 +60,14 @@ anyone and those logged in respectively. If you wish to hide it so that only
 the members of the project or group have access to it, uncheck the **Public
 pipelines** checkbox and save the changes.
 
+## Auto-cancel pending pipelines
+
+> [Introduced][ce-9362] in GitLab 9.1.
+
+If you want to auto-cancel all pending non-HEAD pipelines on branch, when 
+new pipeline will be created (after your git push or manually from UI), 
+check **Auto-cancel pending pipelines** checkbox and save the changes.
+
 ## Badges
 
 In the pipelines settings page you can find pipeline status and test coverage
@@ -111,3 +119,4 @@ into your `README.md`:
 
 [var]: ../../../ci/yaml/README.md#git-strategy
 [coverage report]: #test-coverage-parsing
+[ce-9362]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9362