apply: make init_apply_state() return -1 instead of exit()ing

apply: make init_apply_state() return -1 instead of exit()ing

To libify `git apply` functionality we have to signal errors to the
caller instead of exit()ing.

To do that in a compatible manner with the rest of the error handling
in "builtin/apply.c", init_apply_state() should return -1 instead of
calling exit().

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