Make it clear what happens when MRs that share the same commit are merged
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=262650)
</details>
<!--IssueSummary end-->
<!-- The first section "Release notes" is required if you want to have your release post blog MR auto generated. Currently in BETA, details on the **release post item generator** can be found in the handbook: https://about.gitlab.com/handbook/marketing/blog/release-posts/#release-post-item-generator and this video: https://www.youtube.com/watch?v=rfn9ebgTwKg. The next four sections: "Problem to solve", "Intended users", "User experience goal", and "Proposal", are strongly recommended in your first draft, while the rest of the sections can be filled out during the problem validation or breakdown phase. However, keep in mind that providing complete and relevant information early helps our product team validate the problem and start working on a solution. -->
### Release notes
TBD
### Problem to solve
If two MRs share a commit and a user merges one of those, the other MR will show the user merged it as well, even though no explicit action was taken. This leads to confusion about who did what action and why.
This exact issue happened on a recent MR: https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/64254#note_424541136
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
### Proposal
MR1: User explicitly merged
MR2: User did not explicitly merge
MR1 and MR2 share the same commit
Make it clear what happened on the MR2. A potential solution could be providing a simple message on MR2 that it was merged by way of MR1. Also include language they share the same commmit and cross link if possible.
issue