merge-recursive: change current file dir string_lists to hashmap

merge-recursive: change current file dir string_lists to hashmap

The code was using two string_lists, one for the directories and
one for the files.  The code never checks the lists independently
so we should be able to only use one list.  The string_list also
is a O(log n) for lookup and insertion.  Switching this to use a
hashmap will give O(1) which will save some time when there are
millions of paths that will be checked.

Signed-off-by: Kevin Willford <kewillf@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>