util: implement sortdict.insert()

As flagged by pytype (reported via Matt Harbison, thanks). This was
broken by bd0fd3ff9916 (util: rewrite sortdict using Python 2.7's
OrderedDict, 2017-05-16). We actually call insert() on
namespaces.py:100, but we clearly don't have test coverage of that an
no users have reported it AFAIK.

Differential Revision: https://phab.mercurial-scm.org/D7680

mercurial/util.py

@@ -1253,6 +1253,9 b' class sortdict(collections.OrderedDict):'
     >>> d2.update([(b'a', 2)])
     >>> list(d2.keys()) # should still be in last-set order
     ['b', 'a']
+    >>> d1.insert(1, b'a.5', 0.5)
+    >>> d1
+    sortdict([('a', 0), ('a.5', 0.5), ('b', 1)])
     '''
 
     def __setitem__(self, key, value):
@@ -1268,6 +1271,14 b' class sortdict(collections.OrderedDict):'
             for k, v in src:
                 self[k] = v
 
+    def insert(self, position, key, value):
+        for (i, (k, v)) in enumerate(list(self.items())):
+            if i == position:
+                self[key] = value
+            if i >= position:
+                del self[k]
+                self[k] = v
+
 
 class cowdict(cow, dict):
     """copy-on-write dict