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

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

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", gitdiff_*() functions should return -1 instead
of calling die().

A previous patch made it possible for gitdiff_*() functions to
return -1 in case of error. Let's take advantage of that to
make gitdiff_verify_name() return -1 on error, and to have
gitdiff_oldname() and gitdiff_newname() directly return
what gitdiff_verify_name() returns.

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>