API: Get refs for a particular commit

What does this MR do?

This merge request adds the functionality to retrieve the refs where a particular commit is included.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/18014

changed milestone to %10.3

Thanks for this @razer6! Is this something you'll be willing to finish off or should be take over or close?

added awaiting feedback label

assigned to @razer6

@jameslopez Thanks for reminding me. I'll start rebasing and finishing this MR in the evening :)

removed milestone

added 5791 commits

• 58617553...7534f7a8 - 5789 commits from branch master
• 04000fa4 - Basic ref fetching for commits
• 0b99672e - Start with tests

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added 2 commits

• fe2c1c9a - Basic ref fetching for commits
• f2216ea7 - Start with tests

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marked the checklist item API support added as completed

marked the checklist item Tests added for this feature/bug as completed

added 2 commits

• cea2a8f7 - Basic ref fetching for commits
• 2bbf75f2 - Start with tests

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changed the description

resolved all discussions

assigned to @jameslopez

@jameslopez Can you take a look before I make the docs for that feature?