builtin/apply: make gitdiff_*() return 1 at end of header

builtin/apply: make gitdiff_*() return 1 at end of header

The gitdiff_*() functions that are called as p->fn() in parse_git_header()
should return 1 instead of -1 in case of end of header or unrecognized
input, as these are not real errors. It just instructs the parser to break
out.

This makes it possible for gitdiff_*() functions to return -1 in case of a
real error. This will be done in a following patch.

Helped-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>