merge and reset: adjust for "reset --hard" messages

merge and reset: adjust for "reset --hard" messages

An earlier commit made "reset --hard" chattier but leaking its
message from "git rebase" (which calls it when first rewinding
the current branch to prepare replaying our own changes) without
explanation was confusing, so add an extra message to mention
it.  Inside restorestate in merge (which is rarely exercised
codepath, where more than one strategies are attempted),
resetting to the original state uses "reset --hard" -- this can
be squelched entirely.

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