Auto-set title for branches created from issues
This sets the title for a new MR to 'Resolves "$issue-title"' when:
- The source branch for the MR begins with a value iid.
- The MR has more than one commit in its diff (if there's one commit, keep using the commit's first line).
- The iid does not point to a confidential issue.
Single commit:

Multiple commits:

Confidential issue:

cc @DouweM @zj
Closes#14564
See merge request !3966
If a branch starts with an issue's IID, followed by a hyphen, the
description will be updated to say that is closes the issue. This also
updates the title of the merge request to 'Resolves "$issue-title"', as
long as:
- There is more than one commit in the merge request (if there is only
one commit, the commit's title will be used as before)
- The issue's IID is valid for the project
This is a combination of 3 commits.
- Update the bare repositories info/attributes if the default branch is updated
- Check the diff attributes of a file before showing a diff
- Update CHANGELOG
Ci::Commit becomes a Pipeline object
1. Ci::Commit receives context: ref, :tag.
1. One Ci::Commit describes a one Pipeline
1. Pipeline is created from `.gitlab-ci.yml`
1. Pipeline is a ordered group of builds
1. We test MR against Pipeline
1. Pipelines have a separate view (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3703)
1. Pipeline can be triggered from UI (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3703)
1. Later we change `Trigger -> TriggerRequest -> Build` to `Trigger -> Pipeline` (future)
1. We add a Pipeline Hook that will be triggered on Pipeline status change (future)
1. We extend notifications to use `Pipeline Hook` to send summary on pipeline changes (future)
After merging that I'll prepare a separate MR that will unify naming, database columns, table names:
```
Ci::Commit -> Pipeline
Ci::Build -> Build
CommitStatus -> Job
GenericCommitStatus -> ExternalJob
ci_commits -> pipelines
ci_builds -> jobs
```
This MR implements first 5 points.
This is made to solve this issue https://gitlab.com/gitlab-org/gitlab-ce/issues/14149.
See merge request !3653