Fix reprocessing skipped jobs when retrying pipeline

What does this MR do?

This MR fixes a bug introduced in !9021 (merged) and makes it possible to reprocess jobs that we skipped after a failure in previous stages.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug

What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/26551#note_23929871

mentioned in issue #26551 (closed)

assigned to @ayufan

added 1 commit

• 1965482f - Reprocess jobs in stages once when retrying pipeline

Compare with previous version

mentioned in issue #28530 (moved)

resolved all discussions

added 1 commit

• 830b8d8c - Process skipped jobs in the pipeline when retrying it

Compare with previous version

added 1 commit

• ed986806 - Process skipped jobs in the pipeline when retrying it

Compare with previous version

added 1 commit

• 02699f26 - Add basic specs for optimistic locking mixin

Compare with previous version

added 1 commit

• ca378aa3 - Fix Rubocop offense in OptimisticLocking module

Compare with previous version

enabled an automatic merge when the pipeline for ca378aa3 succeeds

merged