heredoctest: use the same dict for local/global contexts as in doctest

In order to mimic module-level evaluation, globals and locals should be the
same object, so doctest does not pass separate locals dict.

https://docs.python.org/2.7/reference/simple_stmts.html#exec

This fixes NameError in the following example:

    >>> import foo
    >>> def bar():
    ...     foo  # must exist in globalvars

tests/heredoctest.py

@@ -1,7 +1,6 b''
 import sys
 
 globalvars = {}
-localvars = {}
 lines = sys.stdin.readlines()
 while lines:
     l = lines.pop(0)
@@ -14,6 +13,6 b' while lines:'
             snippet += "\n" + l[4:]
         c = compile(snippet, '<heredoc>', 'single')
         try:
-            exec c in globalvars, localvars
+            exec c in globalvars
         except Exception, inst:
             print repr(inst)

tests/test-unified-test.t

@@ -37,6 +37,11 b' Doctest commands:'
   z
   >>> print
   
+  >>> foo = 'global name'
+  >>> def func():
+  ...     print foo, 'should be visible in func()'
+  >>> func()
+  global name should be visible in func()
 
 Regular expressions: