Rename static variable write_index to update_index in builtin-apply.c

Rename static variable write_index to update_index in builtin-apply.c

This is an internal variable used to tell if we need to write
out the resulting index.

I'll be introducing write_index() function which would collide
with it.

Signed-off-by: Junio C Hamano <junkio@cox.net>