stream-clone: factor computation of requirement of a stream clone

This gather code duplicated in multiple place and will make it easier to modify
it safely in the future.

Differential Revision: https://phab.mercurial-scm.org/D12028

mercurial/streamclone.py

@@ -47,6 +47,15 b' def new_stream_clone_requirements('
     return requirements
 
 
+def streamed_requirements(repo):
+    """the set of requirement the new clone will have to support
+
+    This is used for advertising the stream options and to generate the actual
+    stream content."""
+    requiredformats = repo.requirements & repo.supportedformats
+    return requiredformats
+
+
 def canperformstreamclone(pullop, bundle2=False):
     """Whether it is possible to perform a streaming clone as part of pull.
 
@@ -346,7 +355,7 b' def generatebundlev1(repo, compression=b'
     if compression != b'UN':
         raise ValueError(b'we do not support the compression argument yet')
 
-    requirements = repo.requirements & repo.supportedformats
+    requirements = streamed_requirements(repo)
     requires = b','.join(sorted(requirements))
 
     def gen():

mercurial/wireprotov1server.py

@@ -300,7 +300,7 b' def _capabilities(repo, proto):'
     if streamclone.allowservergeneration(repo):
         if repo.ui.configbool(b'server', b'preferuncompressed'):
             caps.append(b'stream-preferred')
-        requiredformats = repo.requirements & repo.supportedformats
+        requiredformats = streamclone.streamed_requirements(repo)
         # if our local revlogs are just revlogv1, add 'stream' cap
         if not requiredformats - {requirementsmod.REVLOGV1_REQUIREMENT}:
             caps.append(b'stream')