dir: respect string length argument of read_directory_recursive()

dir: respect string length argument of read_directory_recursive()

A directory name is passed to read_directory_recursive() as a
length-limited string, through the parameters base and baselen.
Suprisingly, base must be a NUL-terminated string as well, as it is
passed to opendir(), ignoring baselen.

Fix this by postponing the call to opendir() until the length-limted
string is added to a strbuf, which provides a NUL in the right place.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>