superproject: Use bugurl from contactinfo in the missing commits error message.

+ In XmlManifest._Unload set 'bugurl' to Wrapper().BUG_URL.
+ contactinfo returns a namedtuple.
+ bug_url can be accessed as self._manifest.contactinfo.bugurl.

Tested the code with the following commands.

$ ./run_tests -v

Added contactinfo tag to default.xml and verified that bugurl is used.

Bug: [google internal] b/186220520.
Change-Id: Iaafd6465e072b2e47a0a0b548bf6cb608a0b0a04
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/306342
Tested-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

tests/test_manifest_xml.py

@@ -576,7 +576,7 @@ class ContactinfoElementTests(ManifestParseTestCase):
   <contactinfo bugurl="{bugurl}"/>
 </manifest>
 """)
-    self.assertEqual(manifest.contactinfo['bugurl'], bugurl)
+    self.assertEqual(manifest.contactinfo.bugurl, bugurl)
     self.assertEqual(
         manifest.ToXml().toxml(),
         '<?xml version="1.0" ?><manifest>'