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

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

To libify `git apply` functionality we have to signal errors to the
caller instead of die()ing. Let's do that by returning -1 instead of
die()ing in read_patch_file().

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