revert: change help_msg() to take no argument

revert: change help_msg() to take no argument

This is needed because the following commits will make it
possible to cherry-pick many commits instead of just one.

So it will be possible to pass for example ranges of commits
to "git cherry-pick" and this means that it will not be
possible to use the arguments passed to "git cherry-pick" in
the help message.

The help message will have to use the sha1 of the currently
processed commit.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>