lfs: default to not using workers for upload/download

I ran into truncated uploads with this defaulting to on.  Wojciech Lis diagnosed
it as creating keepalive connections prior to forking, and illegally
multiplexing the same connection. [1]  I didn't notice a problem with the couple
of downloads I tried, but disabled both for simplicity and safety.

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-January/109916.html

hgext/lfs/__init__.py

@@ -119,6 +119,9 b' configitem = registrar.configitem(config'
 configitem('experimental', 'lfs.user-agent',
     default=None,
 )
+configitem('experimental', 'lfs.worker-enable',
+    default=False,
+)
 
 configitem('lfs', 'url',
     default=None,

hgext/lfs/blobstore.py

@@ -356,8 +356,13 b' class _gitlfsremote(object):'
                             continue
                         raise
 
-        oids = worker.worker(self.ui, 0.1, transfer, (),
-                             sorted(objects, key=lambda o: o.get('oid')))
+        # Until https multiplexing gets sorted out
+        if self.ui.configbool('experimental', 'lfs.worker-enable'):
+            oids = worker.worker(self.ui, 0.1, transfer, (),
+                                 sorted(objects, key=lambda o: o.get('oid')))
+        else:
+            oids = transfer(sorted(objects, key=lambda o: o.get('oid')))
+
         processed = 0
         for _one, oid in oids:
             processed += sizes[oid]