mmap: backed out changeset 74a9f428227e stable

mmap: backed out changeset 74a9f428227e

There have been concrete and theoretical issues raised, this will need more
work during the next cycle.

--- a/mercurial/configitems.py	Fri Jan 18 23:22:56 2019 -0500
+++ b/mercurial/configitems.py	Fri Jan 18 16:02:26 2019 +0100
@@ -988,7 +988,7 @@
     default=False,
 )
 coreconfigitem('storage', 'mmap-threshold',
-    default='1MB',
+    default=None,
     alias=[('experimental', 'mmapindexthreshold')],
 )
 coreconfigitem('rewrite', 'backup-bundle',