Up to now, path-lists were sorted at all times.  But sometimes it
is much more convenient to build the list and sort it at the end,
or sort it not at all.

Add path_list_append() and sort_path_list() to allow that.

Also, add the unsorted_path_list_has_path() function, to do a linear
search.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>