rerere.autoupdate: change the message when autoupdate is in effect

rerere.autoupdate: change the message when autoupdate is in effect

This changes the message rerere issues after reusing previous conflict
resolution from "Resolved" to "Staged" when autoupdate option is in
effect.

It is envisioned that in practice, some auto resolitions are trickier and
iffier than others, and we would want to add a feature to mark individual
resolutions as "this is ok to autoupdate" or "do not autoupdate the result
using this resolution even when rerere.autoupdate is in effect" in the
future.  When that happens, these messages will make the distinction
clearer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>