Provide way of enforcing conventional commits in MRs
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### Problem to solve
One way of automatically generating changelogs is to ask MRs to include at least one commit in the [conventional commit](https://www.conventionalcommits.org/en/v1.0.0/) format. Unfortunately, this is easily forgotten, and results in a change being lost. It would be great to add a way of encouraging these commits, and reminding developers to include them.
### Intended users
A Development Team Lead would change a repository level setting. Developers would then be presented with new UI when opening and merging an MR.
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### User experience goal
A Development Lead should be confident that all MRs add an entry to the changelog, unless this was specifically decided against.
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### Proposal
I propose the ability to specify a RegEx in the project's settings, which each commit message's first line is matched on. There would also be two settings, specifying which commits this must apply to (at least one/all), and what to do if they don't (warn/block merging).

When opening an MR that does not fulfil the requirement:
<img height=200 src="/uploads/e3ea0bc065405204d7142779fa900d62/image.png">
A similar message would show before merging the MR.
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### Further details
Above is just one idea of how this could be approached. I would certainly be open to suggestions.
If the relevant staff at GitLab are keen to see an MR for this, I may be able to allocate some development time to it.
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### Permissions and Security
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### Documentation
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### Availability & Testing
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### What does success look like, and how can we measure that?
Success would be a large project using this, with developers working on the project adding a changelog commit because of the reminders GitLab gives them. They should not find the UX overbearing.
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### What is the type of buyer?
I believe this should be available to as many people as possible, as repositories of all sizes may wish to catalogue changes.
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### Is this a cross-stage feature?
I don't believe so.
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### Links / references
cc/ @m_gill
issue