ui: remove urllib2 from being imported early

Before this change, urllib2 was brought in when constructing the ui object, and
that added ~5ms on my Linux workstation to the hg startup time for every
command, bringing the time for 'HGRCPATH=/dev/null hg root' from 46ms to 40ms.

Now, we construct a single proxy object per initial ui creation (so that if the
ui is copied they share the object), but defer the actual instantiation of it
and the import of urllib2 until it's needed.

# no-check-commit

mercurial/ui.py

@@ -94,6 +94,24 b' default = %s'
 # pager =""",
 }
 
+
+class httppasswordmgrdbproxy(object):
+    """Delays loading urllib2 until it's needed."""
+    def __init__(self):
+        self._mgr = None
+
+    def _get_mgr(self):
+        if self._mgr is None:
+            self._mgr = urlreq.httppasswordmgrwithdefaultrealm()
+        return self._mgr
+
+    def add_password(self, *args, **kwargs):
+        return self._get_mgr().add_password(*args, **kwargs)
+
+    def find_user_password(self, *args, **kwargs):
+        return self._get_mgr().find_user_password(*args, **kwargs)
+
+
 class ui(object):
     def __init__(self, src=None):
         """Create a fresh new ui object if no src given
@@ -145,7 +163,7 b' class ui(object):'
             # shared read-only environment
             self.environ = encoding.environ
 
-            self.httppasswordmgrdb = urlreq.httppasswordmgrwithdefaultrealm()
+            self.httppasswordmgrdb = httppasswordmgrdbproxy()
 
         allowed = self.configlist('experimental', 'exportableenviron')
         if '*' in allowed:
@@ -172,7 +190,7 b' class ui(object):'
         """Clear internal state that shouldn't persist across commands"""
         if self._progbar:
             self._progbar.resetstate()  # reset last-print time of progress bar
-        self.httppasswordmgrdb = urlreq.httppasswordmgrwithdefaultrealm()
+        self.httppasswordmgrdb = httppasswordmgrdbproxy()
 
     def formatter(self, topic, opts):
         return formatter.formatter(self, topic, opts)