context: drop support for changeid='' (API)

Since the previous commit, there seem to be no users who pass '' to
repo.__getitem__, so let's drop support for it.

It may seem like a small cost to keep support for it, but I've spent
time being confused by it twice already.

Differential Revision: https://phab.mercurial-scm.org/D3021

mercurial/context.py

@@ -411,9 +411,6 b' class changectx(basectx):'
         """changeid is a revision number, node, or tag"""
         super(changectx, self).__init__(repo)
 
-        if changeid == '':
-            changeid = '.'
-
         try:
             if isinstance(changeid, int):
                 self._node = repo.changelog.node(changeid)