Allow to "comment and mark merged" for merge requests / Provide "Merged in $SHA" for MRs

Description

Due to #85 (closed), #895, #1993 (closed), #2891 (closed), ce:#22292, and probably a lot more (all issues that currently prevent to merge GitLab MRs via the UI, especially if fast-forward merges are enabled / desired) it would be helpful if MRs would offer an option to mark them "merged" even if GitLab is not able to detect this fact automatically.

This will for example allow users to checkout the commits of an MR locally, manually rebase and commit them and finally mark the MR accordingly.
Also it will allow project maintainers to modify an MR locally (e.g. some minor formatting changes) before committing and still mark the corresponding MR as "merged".

Right now the only possibility is to mark the MR as "closed" which is wrong, makes it difficult to sort/search MRs properly and is unrewarding for the creator of the MR.

Proposal

Add a possibility to mark merge requests as "merged", even if the original SHAs of the commits never actually end up in the repo.

Two ideas on how this could work:

1. The easiest solution would be to add a second option to the "Close merge request" / "Comment & close merge request" button:
   "Mark merged" / "Comment & mark merged". Instead of marking the MR "closed" this would then simply mark it "Merged"
2. alternatively one could offer an input field "merged in:" where the user could enter commit SHAs of the merged commits that could then be handled exactly like a proper merge (without rebase) would have been handled.