views.main: Show 'Exit Diff Mode' when using Diff->Expression

We weren't showing the 'Exit Diff Mode' button when going into
the 'Diff->Expression' mode.

Closes #31

Signed-off-by: David Aguilar <davvid@gmail.com>

cola/views/main.py

@@ -211,7 +211,9 @@ def _update_view(self):
 
     def _mode_changed(self, mode):
         """React to mode changes; hide/show the "Exit Diff Mode" button."""
-        if mode in (self.model.mode_review, self.model.mode_diff):
+        if mode in (self.model.mode_review,
+                    self.model.mode_diff,
+                    self.model.mode_diff_expr):
             self.alt_button.setMinimumHeight(40)
             self.alt_button.show()
         else: