diff -r 623743010133 -r f134fb33c906 mercurial/streamclone.py
--- a/mercurial/streamclone.py	Fri Oct 02 16:24:56 2015 -0700
+++ b/mercurial/streamclone.py	Fri Oct 02 21:39:04 2015 -0700
@@ -17,6 +17,32 @@
     util,
 )
 
+def maybeperformstreamclone(repo, remote, heads, stream):
+    # now, all clients that can request uncompressed clones can
+    # read repo formats supported by all servers that can serve
+    # them.
+
+    # if revlog format changes, client will have to check version
+    # and format flags on "stream" capability, and use
+    # uncompressed only if compatible.
+
+    if stream is None:
+        # if the server explicitly prefers to stream (for fast LANs)
+        stream = remote.capable('stream-preferred')
+
+    if stream and not heads:
+        # 'stream' means remote revlog format is revlogv1 only
+        if remote.capable('stream'):
+            streamin(repo, remote, set(('revlogv1',)))
+        else:
+            # otherwise, 'streamreqs' contains the remote revlog format
+            streamreqs = remote.capable('streamreqs')
+            if streamreqs:
+                streamreqs = set(streamreqs.split(','))
+                # if we support it, stream in and adjust our requirements
+                if not streamreqs - repo.supportedformats:
+                    streamin(repo, remote, streamreqs)
+
 def allowservergeneration(ui):
     """Whether streaming clones are allowed from the server."""
     return ui.configbool('server', 'uncompressed', True, untrusted=True)