git: add the standard `testedwith` attribute

Otherwise this shows up as an external extension.

Differential Revision: https://phab.mercurial-scm.org/D9435

hgext/git/__init__.py

@@ -29,6 +29,12 b' from . import ('
     index,
 )
 
+# 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'
+
 configtable = {}
 configitem = registrar.configitem(configtable)
 # git.log-index-cache-miss: internal knob for testing