purge: restore the `testedwith` declaration

Otherwise, this extension gets highlighted as a 3rd party extension when an
exception occurs.

Differential Revision: https://phab.mercurial-scm.org/D10895

hgext/purge.py

@@ -30,3 +30,9 b' version 5.7. Please use :hg:`purge ...` '
 
 # This empty extension looks pointless, but core mercurial checks if it's loaded
 # to implement the slightly different behavior documented above.
+
+# 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 = b'ships-with-hg-core'