stream-clone: bail-out earlier if destination repo is not empty

The `canperformstreamclone` function is bit messy. However it seems clearer to
me to process the very generic condition about "can we consider a stream-clone
at all", before checking if a stream-clone is requested and if a compatible
protocol can be negotiated with the server.

So I am doing some gratuitous movement so reorder conditional.

mercurial/streamclone.py

@@ -69,6 +69,10 b' def canperformstreamclone(pullop, bundle'
     repo = pullop.repo
     remote = pullop.remote
 
+    # Streaming clone only works on an empty destination repository
+    if len(repo):
+        return False, None
+
     bundle2supported = False
     if pullop.canusebundle2:
         if b'v2' in pullop.remotebundle2caps.get(b'stream', []):
@@ -84,10 +88,6 b' def canperformstreamclone(pullop, bundle'
     elif bundle2 and not bundle2supported:
         return False, None
 
-    # Streaming clone only works on empty repositories.
-    if len(repo):
-        return False, None
-
     # Streaming clone only works if all data is being requested.
     if pullop.heads:
         return False, None