namespaces: generate template keyword when registering a namespace

For any namespace, we generate a template keyword. For example, given a
namespace 'babar', we automatically have the ability to use it in a template:

hg log -r . -T '{babars % "King: {babar}\n"}'

Furthermore, we only generate this keyword for a namespace if one doesn't
already exist. This is necessary for 'branches' and 'bookmarks' since both of
those have concepts of 'current' (something outside the namespace api) and also
allows extensions to override default behavior if desired.

mercurial/namespaces.py

@@ -1,5 +1,6 b''
 from i18n import _
 from mercurial import util
+import templatekw
 
 def tolist(val):
     """
@@ -74,6 +75,13 b' class namespaces(object):'
         else:
             self._names[namespace] = val
 
+        # we only generate a template keyword if one does not already exist
+        if namespace not in templatekw.keywords:
+            def generatekw(**args):
+                return templatekw.shownames(namespace, **args)
+
+            templatekw.keywords[namespace] = generatekw
+
     def singlenode(self, repo, name):
         """
         Return the 'best' node for the given name. Best means the first node