phabricator: add testedwith boilerplate

phabricator: add testedwith boilerplate

--- a/hgext/phabricator.py	Thu Sep 20 12:13:00 2018 -0700
+++ b/hgext/phabricator.py	Sat Sep 15 16:35:39 2018 +0900
@@ -71,6 +71,12 @@
     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)