configitems: change the `verify.skipflags` default value to avoid a py3 crash

The revlog and LFS modules use various `&` and `&=` operations with this value,
which no longer treats `None` as 0.  Since nothing cares if it was actually set
in the config or not, just default to 0 for simplicity.

mercurial/configitems.py

@@ -2543,7 +2543,7 b' coreconfigitem('
 coreconfigitem(
     b'verify',
     b'skipflags',
-    default=None,
+    default=0,
 )
 coreconfigitem(
     b'web',