clone: use dir-iterator to avoid explicit dir traversal
Replace usage of opendir/readdir/closedir API to traverse directories
recursively, at copy_or_link_directory function, by the dir-iterator
API. This simplifies the code and avoids recursive calls to
copy_or_link_directory.
This process also makes copy_or_link_directory call die() in case of an
error on readdir or stat inside dir_iterator_advance. Previously it
would just print a warning for errors on stat and ignore errors on
readdir, which isn't nice because a local git clone could succeed even
though the .git/objects copy didn't fully succeed.
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>