narrow: move changegroup.supportedoutgoingversions() override to core

Also document why we need cg3 or higher.

Differential Revision: https://phab.mercurial-scm.org/D2488

hgext/narrow/narrowchangegroup.py

@@ -21,16 +21,6 b' from mercurial import ('
 
 def setup():
 
-    def supportedoutgoingversions(orig, repo):
-        versions = orig(repo)
-        if changegroup.NARROW_REQUIREMENT in repo.requirements:
-            versions.discard('01')
-            versions.discard('02')
-        return versions
-
-    extensions.wrapfunction(changegroup, 'supportedoutgoingversions',
-                            supportedoutgoingversions)
-
     def prune(orig, self, revlog, missing, commonrevs):
         if isinstance(revlog, manifest.manifestrevlog):
             matcher = getattr(self._repo, 'narrowmatch',

mercurial/changegroup.py

@@ -898,6 +898,11 b' def supportedoutgoingversions(repo):'
         # support versions 01 and 02.
         versions.discard('01')
         versions.discard('02')
+    if NARROW_REQUIREMENT in repo.requirements:
+        # Versions 01 and 02 don't support revlog flags, and we need to
+        # support that for stripping and unbundling to work.
+        versions.discard('01')
+        versions.discard('02')
     return versions
 
 def localversion(repo):