builtin/apply: avoid parameter shadowing 'p_value' global

builtin/apply: avoid parameter shadowing 'p_value' global

Let's just rename the global 'state_p_value' as it will become
'state->p_value' in a following patch.

This also avoid errors when compiling with -Wshadow and makes
it safer to later move global variables into a "state" struct.

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