Empty rebases should not generate a pipeline

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Summary

When the merge request rebase REST API is used, every invocation done generates a new pipeline.

A pipeline is generated even if the rebase itself was a no-operation (target had no new changes).

Repeating the /rebase API keeps generating new pipelines, all for the same ref, which wastes resources.

This is incorrect behavior as an empty push from the CLI, as comparison, does not generate a new pipeline.

The API does provide a skip_ci option to explicitly skip the pipeline via a push option set internally, but the default behavior appears incorrect when the rebase does no real work/change.

Steps to reproduce

1. Open an MR on any project with a pipeline
2. Run the PUT Rebase API on the MR repeatedly (4-5 times): https://docs.gitlab.com/ee/api/merge_requests.html#rebase-a-merge-request
3. Observe the MR's pipelines tab for duplicated, and un-cancelled pipelines

Example Project

https://gitlab.com/gitlab-gold/hchouraria/rebase-redundant-pipeline/-/merge_requests/1/pipelines

What is the current bug behavior?

Rebase with no change generates a pipeline.

What is the expected correct behavior?

Rebase with no change does not generate a pipeline.

(Or,)

New rebase generated pipelines must cancel previously running redundant pipelines, even if they are for the same ref.

Relevant logs and/or screenshots

https://gitlab.com/gitlab-gold/hchouraria/rebase-redundant-pipeline/-/merge_requests/1/pipelines

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

The CI run here is unconditional, unless entirely skipped: https://gitlab.com/gitlab-org/gitlab/blob/v13.11.0-ee/app/models/repository.rb#L1050-1068