os: avoid allocating a string for ReadDir skipped entries on Windows

os: avoid allocating a string for ReadDir skipped entries on Windows

Shave off a few allocations while reading a directory by checking
if the entry name is "." or ".." before allocating a string for it.

Change-Id: I05a87d7572bd4fc191db70aaa9e22a6102f68b4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/520415
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>