editor: Fix overwriting mode side effects

- insert_text normally shouldn't always overwrite when in
  overwriting mode.

diff_edit/editor.py

@@ -339,10 +339,10 @@ class Editor:
             self.text_widget[start_y:end_y+1] = [new_line]
             self.cursor_x, self.cursor_y = start_x, start_y
 
-    def insert_text(self, text):
+    def insert_text(self, text, is_overwriting=False):
         try:
             current_line = self.text_widget[self.cursor_y]
-            replace_count = len(text) if self.is_overwriting else 0
+            replace_count = len(text) if is_overwriting else 0
             self.text_widget[self.cursor_y] = (current_line[:self.cursor_x] + text
                                                + current_line[self.cursor_x+replace_count:])
         except IndexError:
@@ -575,7 +575,7 @@ class Editor:
             except IndexError:
                 self.ring_bell()
         elif term_code in self._PRINTABLE:
-            self.insert_text(term_code)
+            self.insert_text(term_code, is_overwriting=self.is_overwriting)
         else:
             self.insert_text(repr(term_code))
         self.previous_term_code = term_code

tests/editor_test.py

@@ -83,11 +83,11 @@ class EditorTestCase(unittest.TestCase):
         # overwrite
         self.editor.toggle_overwrite()
         self.editor.cursor_left()
-        self.editor.insert_text("d")
+        self.editor.insert_text("d", is_overwriting=True)
         self._assert_editor("abd", (3, 0))
         self.editor.cursor_left()
         self.editor.cursor_left()
-        self.editor.insert_text("ef")
+        self.editor.insert_text("ef", is_overwriting=True)
         self._assert_editor("aef", (3, 0))
 
     def test_enter(self):