add is_dot_or_dotdot inline function

add is_dot_or_dotdot inline function

A new inline function is_dot_or_dotdot is used to check if the
directory name is either "." or "..". It returns a non-zero value if
the given string is "." or "..". It's applicable to a lot of Git
source code.

Signed-off-by: Alexander Potashev <aspotashev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>