Fix wxFileName::MakeRelativeTo() for directory relatively to itself.

The expected result in this case is ".", but the filename became empty instead
when wxPATH_NATIVE was used.

Fix this by examining GetFormat(format), which takes care of mapping
wxPATH_NATIVE to its real value, instead of wxPATH_NATIVE itself.

Also add a unit test verifying that this works as expected.

Closes #17010.

docs/changes.txt

@@ -577,6 +577,10 @@ Major new features in this release
 3.0.3: (released 2015-??-??)
 ----------------------------
 
+All:
+
+- Fix wxFileName::MakeRelativeTo() for directory relatively to itself.
+
 Unix:
 
 - Fix wxIPaddress::Hostname() failing if /etc/hosts contained very long names.