builtin/apply: make create_file() return -1 on error

builtin/apply: make create_file() return -1 on error

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", create_file() should just return what
add_conflicted_stages_file() and add_index_file() are returning
instead of calling exit().

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