unpack-trees: keep gently check inside add_rejected_path

unpack-trees: keep gently check inside add_rejected_path

This basically follows the footsteps of 6a143aa2b2 (checkout -m:
attempt merge when deletion of path was staged - 2014-08-12) where
there gently check is moved inside reject_merge() so that callers do
not accidentally forget it.

add_rejected_path() has the same usage pattern. All call sites check
gently first, then decide to call add_rejected_path() if needed. Move
the check inside.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>