diff-no-index: rename read_directory()

diff-no-index: rename read_directory()

In the next patch, we will replace a manual checking of "." or ".."
with a call to is_dot_or_dotdot() defined in dir.h.  The private
function read_directory() defined in this file will conflict with
the global function declared there when we do so.

As a preparatory step, rename the private read_directory() to avoid
the name collision.

Helped-by: Eric Sunshine <redacted>
Signed-off-by: Brian Bourn <redacted>
Signed-off-by: Junio C Hamano <redacted>