apply: change error_routine when silent

apply: change error_routine when silent

To avoid printing anything when applying with
`state->apply_verbosity == verbosity_silent`, let's save the
existing warn and error routines before applying, and let's
replace them with a routine that does nothing.

Then after applying, let's restore the saved routines.

Note that, as we need to restore the saved routines in all
cases, we cannot return early any more in apply_all_patches().

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