builtin/apply: make parse_ignorewhitespace_option() return -1 instead of die()ing

builtin/apply: make parse_ignorewhitespace_option() return -1 instead of die()ing

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

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

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