Squashing a merge request should leave a system note
## Description When a merge request has been merged with a squash option, there is no indication of this fact in the user interface. Without a system note / something else it is very difficult to know when a merge request has been squashed before merge. I sometimes browse commits history / use my Git-fu when I see that `git branch --merged` does not show my branch, even when I'm sure a merge request has been merged already. /cc @smcgivern @jacobvosmaer\-gitlab
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