dir: convert to strbuf

dir: convert to strbuf

The functions read_directory_recursive() and treat_leading_path() both
use buffers sized to fit PATH_MAX characters.  The latter can be made to
overrun its buffer, e.g. like this:

	$ a=0123456789abcdef
	$ a=$a$a$a$a$a$a$a$a
	$ a=$a$a$a$a$a$a$a$a
	$ a=$a$a$a$a$a$a$a$a
	$ git add $a/a

Instead of trying to add a check and potentionally forgetting to address
similar cases, convert the involved functions and their helpers to use
struct strbuf.  The patch is suprisingly large because the helpers
treat_path() and treat_one_path() modify the buffer as well and thus need
to be converted, too.

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