merge: don't document non-existing --compact-summary argument
3a54f5bd5d (merge/pull: add the "--compact-summary" option, 2025-06-12)
added the option --compact-summary to both merge and pull. It takes no
no argument, but for merge it got an argument help string. Remove it,
since it is unnecessary.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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