builtin/apply: avoid local variable shadowing 'len' parameter

builtin/apply: avoid local variable shadowing 'len' parameter

This is just a cleanup to avoid errors when compiling with -Wshadow and
to make it safer to later move global variables into a "state" struct.

Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>