fix: Incorrect v-position for parent centered windows

If a user had a secondary monitor above the primary monitor,
and the main windows was opened on the secondary monitor,
child windows that persist their locations would get opened on
the primary monitor.

The fix places new windows at the center of their parent.

Author: RussKie

GitUI/GitExtensionsForm.cs

@@ -139,7 +139,7 @@ protected virtual void RestorePosition()
                 // Calculate location for modal form with parent
                 Location = new Point(
                     Owner.Left + (Owner.Width / 2) - (Width / 2),
-                    Math.Max(0, Owner.Top + (Owner.Height / 2) - (Height / 2)));
+                    Owner.Top + (Owner.Height / 2) - (Height / 2));
             }
 
             if (WindowState != position.State)

UnitTests/GitUITests/GitExtensionsFormTests.cs

@@ -142,12 +142,14 @@ public void RestorePosition_should_scale_sizable_window_if_different_dpi(int sav
             form.Size.Should().Be(new Size(expectedHeightAt96dpi, expectedHeightAt96dpi));
         }
 
-        [Test]
-        public void RestorePosition_should_position_window_with_Owner_set_and_CenterParent()
+        [TestCase(-1000, 100, /* -1000 + (800 - 300)/2 */ -750, /* 100 + (600-200)/2 */300)]
+        [TestCase(0, 0, /* 0 + (800 - 300)/2 */ 250, /* 0 + (600-200)/2 */200)]
+        [TestCase(1000, -400, /* 1000 + (800 - 300)/2 */ 1250, /* -400 + (600-200)/2 */ -200)]
+        public void RestorePosition_should_position_window_with_Owner_set_and_CenterParent(int ownerFormTop, int ownerFormLeft, int expectFormTop, int expectedFormLeft)
         {
             var owner = new Form
             {
-                Location = new Point(-1000, 100),
+                Location = new Point(ownerFormTop, ownerFormLeft),
                 Size = new Size(800, 600)
             };
             var form = new MockForm(true)
@@ -169,7 +171,7 @@ public void RestorePosition_should_position_window_with_Owner_set_and_CenterPare
 
             form.InvokeRestorePosition();
 
-            form.Location.Should().Be(new Point(/* -1000 + (800 - 300)/2 */-750, /* 100 + (600-200)/2 */300));
+            form.Location.Should().Be(new Point(expectFormTop, expectedFormLeft));
         }
 
         private class MockForm : GitExtensionsForm