sourceview: Fix text-wrapping reset from commit dialog (bgo#741620)

Because we set the wrap-mode forcefully in the commit dialog's
sourceview, the gsettings binding was overriding the actual set
preference.

Author: kaiw

data/ui/vcview.ui

@@ -277,7 +277,7 @@
                             <property name="can_focus">True</property>
                             <property name="shadow_type">in</property>
                             <child>
-                              <object class="MeldSourceView" id="textview">
+                              <object class="CommitMessageSourceView" id="textview">
                                 <property name="visible">True</property>
                                 <property name="can_focus">True</property>
                                 <property name="wrap_mode">word</property>

meld/sourceview.py

@@ -96,3 +96,14 @@ def get_y_for_line_num(self, line):
 
     def get_line_num_for_y(self, y):
         return self.get_line_at_y(y)[0].get_line()
+
+
+class CommitMessageSourceView(GtkSource.View):
+
+    __gtype_name__ = "CommitMessageSourceView"
+
+    __gsettings_bindings__ = (
+        ('indent-width', 'tab-width'),
+        ('insert-spaces-instead-of-tabs', 'insert-spaces-instead-of-tabs'),
+        ('draw-spaces', 'draw-spaces'),
+    )