streamclone: don't support stream clone unless repo feature present

This change means custom repository types must opt in to enabling
stream clone. This seems reasonable, as stream clones are a very
low-level feature that has historically assumed the use of revlogs
and the layout of .hg/ that they entail.

Differential Revision: https://phab.mercurial-scm.org/D4853

mercurial/streamclone.py

@@ -18,6 +18,7 b' from . import ('
     error,
     phases,
     pycompat,
+    repository,
     store,
     util,
 )
@@ -178,6 +179,9 b' def maybeperformlegacystreamclone(pullop'
 
 def allowservergeneration(repo):
     """Whether streaming clones are allowed from the server."""
+    if repository.REPO_FEATURE_STREAM_CLONE not in repo.features:
+        return False
+
     if not repo.ui.configbool('server', 'uncompressed', untrusted=True):
         return False