git-commit --amend -m "..." complains?!?

I get

Option -m cannot be combined with -c/-C/-F/--amend.

but that makes no sense: of course there is ample reason for providing
an amended commit message on the command line.  -c, -C and -F indeed
all provide an alternative commit message, but --amend doesn't.

So what is the beef?

-- 
David Kastrup