context: change default changeid from old form '' to '.'

I don't think I've seen repo[''] in the codebase at least in the last
two years.

Differential Revision: https://phab.mercurial-scm.org/D2964

mercurial/context.py

@@ -415,7 +415,7 b' class changectx(basectx):'
     """A changecontext object makes access to data related to a particular
     changeset convenient. It represents a read-only context already present in
     the repo."""
-    def __init__(self, repo, changeid=''):
+    def __init__(self, repo, changeid='.'):
         """changeid is a revision number, node, or tag"""
 
         # since basectx.__new__ already took care of copying the object, we