Missing ability to pre-query branches deleted by "delete merged branches" API endpoint

Proposal

Delete merged branches API endpoint doesn't have any "preview" or "dry run" counterpart in API as far as I can tell, so if one wants to evaluate what is to be deleted - the only way I can see it "kind" of being done is: query for all merged MRs, query existing branches, cross-reference the two manually and use intersection as the resulting list. But that's a rather roundabout way and not necessarily aligned with inner works of Delete merged branches API.

Proposal is to add a way to "preview" this destructive transaction either by providing additional endpoint or by extending Delete merged branches API with dry_run option.