ParentDir and CurrentDir segments in the base aren't always correctly handled

[woody77]

Once the loop hits this arm of the match, it will ignore any ParentDir segments in the base path:

pathdiff/src/lib.rs

Line 66 in a78acde

for _ in itb {

The following fails:

let path = "some/path";
assert_eq!(pathdiff::diff_paths(path, "some/foo/baz/path"), PathBuf::from("../../../path"));

// this fails and returns "../../../../../path" instead.
assert_eq!(pathdiff::diff_paths(path, "some/foo/bar/../baz/path"), PathBuf::from("../../../path"));

[Manishearth]

Yeah I'd accept a PR fixing this