merge-recursive: add get_directory_renames()

merge-recursive: add get_directory_renames()

This populates a set of directory renames for us.  The set of directory
renames is not yet used, but will be in subsequent commits.

Note that the use of a string_list for possible_new_dirs in the new
dir_rename_entry struct implies an O(n^2) algorithm; however, in practice
I expect the number of distinct directories that files were renamed into
from a single original directory to be O(1).  My guess is that n has a
mode of 1 and a mean of less than 2, so, for now, string_list seems good
enough for possible_new_dirs.

Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>