Allow reviewer to suggest a commit message when suggesting changes
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# What
There are currently three options to commit suggested changes:
* Use the default commit message
* Configure a [message template](https://docs.gitlab.com/ee/user/project/merge_requests/commit_templates.html)
* Let the **author** of the MR customize the commit message
I would like to propose a fourth option:
* Let the **reviewer** customize the commit message and offer this to the author as part of the suggestion
# Why
Often times, when applying suggested changes
* Authors might want to get through quickly and therefore might tend to take the default that is offered to them
* Authors already need to wrap their head around what the reviewer is suggesting while the reviewer already knows what the change is about when they are done writing their suggestion
# Mockups
\
(The syntax is just an idea)


# Furthermore
* Technically, we could allow _any_ argument of `git commit` (like `--fixup` (#325951), `--squash`, `--trailer` (this could be useful for #526258))
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