Create a table joining commit SHAs to MRs

We currently serialise the commits present in an MR version in the merge_request_diffs.st_commits column. This is fine for storing and rendering the commit objects for a single MR, but makes it hard to find:

• Which MRs have a commit present now.
• Which MRs had a commit in the past.
• What the previous commits in an MR were. (We have some kind of workaround for this: https://gitlab.com/gitlab-org/gitlab-ce/blob/v9.0.0/app/models/merge_request.rb#L824-834.)

If we had a table joining commits to MRs, or MR diffs, then we could query on it, which would help with:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/2383 - Add link on commit page to merge request that introduced that commit
• https://gitlab.com/gitlab-org/gitlab-ce/issues/28982 - Add Merge request (MR) link in a pipeline email
• https://gitlab.com/gitlab-org/gitlab-ce/issues/29939 - ETag caching for merge request notes polling
• https://gitlab.com/gitlab-org/gitlab-ce/issues/24323 - Don't lose commit-related comments on rebase