bundle2: add a way to add parts during a `getbundle` request

We add a hook function to let extensions add parts to the bundle.

mercurial/exchange.py

@@ -691,8 +691,15 b' def getbundle(repo, source, heads=None, '
     bundler = bundle2.bundle20(repo.ui, b2caps)
     part = bundle2.bundlepart('b2x:changegroup', data=cg.getchunks())
     bundler.addpart(part)
+    _getbundleextrapart(bundler, repo, source, heads=None, common=None,
+                        bundlecaps=None, **kwargs)
     return util.chunkbuffer(bundler.getchunks())
 
+def _getbundleextrapart(bundler, repo, source, heads=None, common=None,
+                        bundlecaps=None, **kwargs):
+    """hook function to let extensions add parts to the requested bundle"""
+    pass
+
 class PushRaced(RuntimeError):
     """An exception raised during unbundling that indicate a push race"""