When multiple patches are passed to git-apply, it will attempt
to open multiple file descriptors to an index, which means
multiple entries will be in the circular cache_file_list.
This change makes git-apply only open the index once and
write the index at exit.
Signed-off-by: Eric Wong <normalperson@yhbt.net> Signed-off-by: Junio C Hamano <junkio@cox.net>