Merge request can have `head_pipeline` instead of or along with `pipeline`

Summary

According to the sample result in GitLab API, merge request can have a pipeline field. In some cases, the merge request also has an undocumented head_pipeline that does not match the pipeline. And the pipeline may not match the pipeline in the MR as displayed by the UI. It can be null or refer to a different pipeline (seen this case only once, the first time I tried to recreate this issue on https://gitlab.com/). The only time when the GitLab API shows a head_pipeline in an MR is when listing merge requests related to issue.

Steps to reproduce

1. Create an MR.
2. Post first commit pending status on a sha: /statuses/<sha>?ref=<branch>&name=name1&target_url=<targetUrl>&state=pending&pipeline_id=-1.
3. Get the sha commit: /repository/commits/<sha> and extract pipeline 1 last_pipeline.id.
4. Post second commit pending status on a sha: /statuses/<sha>?ref=<branch>&name=name2&target_url=<targetUrl>&state=pending&pipeline_id=-1.
5. Get the sha commit: /repository/commits/<sha> and extract pipeline 2 last_pipeline.id.
6. Post commit success status on first pipeline: /statuses/<sha>?ref=<branch>&name=name1&target_url=<targetUrl>&state=success&pipeline_id=<pipeline1>.
7. Post commit failed status on second pipeline: /statuses/<sha>?ref=<branch>&name=name1&target_url=<targetUrl>&state=failed&pipeline_id=<pipeline2>.

Example Project

https://gitlab.com/GeorgesZ473/Issue356086

What is the current bug behavior?

With GitLab Enterprise Edition 13.12.10-ee, the pipeline field is null, while the head_pipeline field seems to correspond with the correct pipeline as display in the MR UI.

With https://gitlab.com/, the pipeline field seems to correspond to the pipeline of the commit, while the head_pipeline seems to correspond to the pipeline of the MR.

What is the expected correct behavior?

The MR should have a pipeline field that match the pipeline displayed in the MR and should not have a head_pipeline field.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com.

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes