infinitepush: opt out of changegroup3 unless explicitly configured

This is currently a no-op, as changegroup3 is disabled by default.  But when it
is enabled, it changes the hash names of the bundle files.  As I don't use this
extension, I have no idea if that's OK or not.  So keep the current default
behavior until we can get more info from actual users, while allowing them to
opt-in for testing purposes.

hgext/infinitepush/__init__.py

@@ -330,6 +330,11 b' def extsetup(ui):'
         clientextsetup(ui)
 
 
+def uipopulate(ui):
+    if not ui.hasconfig(b"experimental", b"changegroup3"):
+        ui.setconfig(b"experimental", b"changegroup3", False, b"infinitepush")
+
+
 def commonsetup(ui):
     wireprotov1server.commands[b'listkeyspatterns'] = (
         wireprotolistkeyspatterns,