combine_path: Fixed corner case.

combine_path() on strings starting with "./../" used to gain one
directory level. Eg:

  Old behavior:
    > combine_path(".", "../foo");
    (1) Result: "../foo"
    > combine_path("./..", "foo");
    (2) Result: "foo"
    > combine_path("./../foo");
    (3) Result: "foo"

  New (fixed) behavior:
    > combine_path(".", "../foo");
    (1) Result: "../foo"
    > combine_path("./..", "foo");
    (2) Result: "../foo"
    > combine_path("./../foo");
    (3) Result: "../foo"

Fixes [PIKE-137].