GraphQL add commits endpoint

Lucas Zampierirequested to merge
lzampier/gitlab:graphql_add_commit into master

What does this MR do and why?

Add MergeRequest Commit endpoint to GraphQL API.
Currently, the GraphQL API offers only one endpoint to access commits by CommitsWithoutMergeCommits, this MR adds a second endpoint called Commits which will not filter for any merge commits.

Related Issues

Closes POV-103

Edited by Lucas Zampieri

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