cherry-pick/revert: error-help message rewording.

cherry-pick/revert: error-help message rewording.

It said "after fixing up, commit the result using -F .msg", but
it was not clear for new people how "fix up" should be done.
Hint "git-update-index <path>".

We could recommend "git commit -a -F .msg" instead, but I am
hesitant to give that suggestion in the blind -- you could do a
cherry-pick, revert or a merge in general in a dirty working
tree as long as local modifications do not overlap with the
merge, but using "commit -a" would include them in the result.

Signed-off-by: Junio C Hamano <junkio@cox.net>