Cherry-picking a merged MR fails, when the content changed by the MR was also updated in the default branch
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### Summary
One of our customers reported that cherry-picking a merged MR fails, when the content changed by the MR was also updated in the default branch
### Steps to reproduce
- create a new branch of a project (let's call it `new-branch-to-pick`)
- make a commit to the branch `new-branch-to-pick` and create an MR to master branch
- merge this MR to master
- switch to the master branch, make commit by changing the same string that was changed in the previously made commit
- go to the merged MR, click cherry-pick to master, you will see the message:
`Sorry, we cannot cherry-pick this merge request automatically. This merge request may already have been cherry-picked, or a more recent commit may have updated some of its content.`
### Example Project
Reproduced with the test project https://gitlab.com/atanayno/learn-rails/
MR that I tried to cherry-pick: https://gitlab.com/atanayno/learn-rails/merge_requests/1
### What is the current behavior?
The message `Sorry, we cannot cherry-pick this merge request automatically. This merge request may already have been cherry-picked, or a more recent commit may have updated some of its content.` is shown.
### What is the expected behavior?
There should probably be an ability to resolve such case via UI: create MR with conflicts.
### Relevant logs and/or screenshots
Screenshot attached
### Additional info
As a workaround, it is possible to do cherry-picking manually in a local repository:
1. create a cherry-pick branch based on master
2. cherry-pick the merge commit made by the merged merge request into the cherry-pick branch. This will result in a conflict.
3. resolve this conflicts, commit the conflict resolution and merge the cherry-pick branch into master via a new merge request.
Similar question was also asked a couple of times at stack overflow:
https://stackoverflow.com/questions/46322142/creation-of-merge-request-in-gitlab-fails
https://stackoverflow.com/questions/49670336/gitlab-sorry-we-cannot-cherry-pick-this-merge-request-automatically
### Output of checks
This issue happens on GitLab.com.
#### Results of GitLab environment info
n/a
#### Results of GitLab application Check
n/a
/cc @smcgivern
issue