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.

tests/filename/filenametest.cpp

@@ -133,6 +133,7 @@ private:
         CPPUNIT_TEST( TestSetPath );
         CPPUNIT_TEST( TestStrip );
         CPPUNIT_TEST( TestNormalize );
+        CPPUNIT_TEST( TestRelative );
         CPPUNIT_TEST( TestReplace );
         CPPUNIT_TEST( TestGetHumanReadable );
 #ifdef __WINDOWS__
@@ -158,6 +159,7 @@ private:
     void TestSplit();
     void TestSetPath();
     void TestStrip();
+    void TestRelative();
     void TestNormalize();
     void TestReplace();
     void TestGetHumanReadable();
@@ -441,6 +443,21 @@ void FileNameTestCase::TestNormalize()
 #endif // __WINDOWS__
 }
 
+void FileNameTestCase::TestRelative()
+{
+    wxFileName fn("a/b.cpp");
+    fn.MakeRelativeTo("a");
+    CPPUNIT_ASSERT_EQUAL( "b.cpp", fn.GetFullPath() );
+
+    fn.AssignDir("a/b");
+    fn.MakeRelativeTo("a");
+    CPPUNIT_ASSERT_EQUAL( "b/", fn.GetFullPath() );
+
+    fn.AssignDir("a");
+    fn.MakeRelativeTo("a");
+    CPPUNIT_ASSERT_EQUAL( "./", fn.GetFullPath() );
+}
+
 void FileNameTestCase::TestReplace()
 {
     static const struct FileNameTest