feat(dot): support filename extension .gv

The dot command from graphviz used .dot for the filename extension in
the past, but .gv is the preferred extension.

References:

#1109

completions/dot

@@ -39,7 +39,7 @@ _comp_cmd_dot()
         return
     fi
 
-    _comp_compgen_filedir dot
+    _comp_compgen_filedir '@(gv|dot)'
 } &&
     complete -F _comp_cmd_dot dot
 

test/t/test_dot.py

@@ -5,3 +5,11 @@ class TestDot:
     @pytest.mark.complete("dot ")
     def test_1(self, completion):
         assert completion
+
+    @pytest.mark.complete("dot t", cwd="dot")
+    def test_2(self, completion):
+        assert completion == ["test1.gv", "test2.dot"]
+
+    @pytest.mark.complete("dot test1", cwd="dot")
+    def test_3(self, completion):
+        assert completion == ".gv"