Fix lexically_relative return when base path evaluates to *this

[vgeorgiev]

Fix the return of lexally_relative when called with the following values:

"/a/b/c".lexally_relative("/a/b/c/d/..") == "."

The proposed change is based on the following line from the documentation:

"otherwise, if N = 0 and a == end() || a->empty(), returns path("."), "

[gulrak]

Nice find! Thank you for the contribution!