Shorten default merge commit title

[fregante]

When merging PRs with a merge commit, GitHub suggests this title:

Merge pull request #${number} from ${user}/${branch}

I suggest shortening this default to:

Merge PR #${number}

A similar feature, sync-pr-commit-title does something similar but just when squashing commits. This new feature could be named clean-merge-commit-title or shorten-merge-commit-title.

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This issue replaces/follows #2768