streamclone: avoid some obscure error in a corner case

I don't really know how, but I ran into this error:

    $ hg clone --stream ssh://user@dummy/empty-repo local-empty-repo
    streaming all changes
    abort: unable to apply stream clone: unsupported format:
    [255]

I think you need an empty list of requirements for this to happen,
which is weird, but an obscure error like this is not exactly helpful
either. Since this is the result of an encoding bug anyway, just fix
it.

Differential Revision: https://phab.mercurial-scm.org/D12402

mercurial/bundle2.py

@@ -2528,7 +2528,8 b' def bundle2getvars(op, part):'
 @parthandler(b'stream2', (b'requirements', b'filecount', b'bytecount'))
 def handlestreamv2bundle(op, part):
 
-    requirements = urlreq.unquote(part.params[b'requirements']).split(b',')
+    requirements = urlreq.unquote(part.params[b'requirements'])
+    requirements = requirements.split(b',') if requirements else []
     filecount = int(part.params[b'filecount'])
     bytecount = int(part.params[b'bytecount'])
 

tests/test-clone-stream.t

@@ -817,3 +817,9 b' Clone non-publishing with obsolescence'
   $ killdaemons.py
 
 #endif
+
+Cloning a repo with no requirements doesn't give some obscure error
+
+  $ mkdir -p empty-repo/.hg
+  $ hg clone -q --stream ssh://user@dummy/empty-repo empty-repo2
+  $ hg --cwd empty-repo2 verify -q