phabricator: add testedwith boilerplate

hgext/phabricator.py

@@ -71,6 +71,12 b' from mercurial.utils import ('
     stringutil,
 )
 
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = 'ships-with-hg-core'
+
 cmdtable = {}
 command = registrar.command(cmdtable)