Display conflict-free merge messages for pull requests (#15773)

Display conflict-free merge messages for pull requests (#15773)

Repositories using external issue tracker tend to use numeric issues in
commits. To prevent conflicts during issue reference parsing or inside
commit hooks, this change respects these configuration and uses the !
character to refer to pull requests in merge commit messages.

For repositories using squash merges, this was already handled.

Signed-off-by: JustusBunsi <61625851+justusbunsi@users.noreply.github.com>
Co-authored-by: zeripath <art27@cantab.net>