apply: move libified code from builtin/apply.c to apply.{c,h}

As most of the apply code in builtin/apply.c has been libified by a number of
previous commits, it can now be moved to apply.{c,h}, so that more code can
use it.

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

Author: chriscool