commitmsg: disable the "amend" action when merging

Make the amend action sensiive to in-progress merges.  The action is now
disabled to prevent users from seeing the "do not amend while merging"
message.

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

Author: davvid

cola/widgets/commitmsg.py

@@ -329,8 +329,10 @@ def setFont(self, font):
         self.description.setFont(font)
 
     def set_mode(self, mode):
+        can_amend = not self.model.is_merging
         checked = (mode == self.model.mode_amend)
         blocksignals = self.amend_action.blockSignals(True)
+        self.amend_action.setEnabled(can_amend)
         self.amend_action.setChecked(checked)
         self.amend_action.blockSignals(blocksignals)