dispatch: disable demandimport for the --debugger option

Something in Python 2.7.9 or so broke the --debugger option with
ui.debugger = ipdb. I get the traceback below. There is some apparent
confusion with demandimport. This should be disabled anyway for the
--debugger option. The debugger must be imported right away, before
any other dispatch. There's no benefit in delaying the debugger
import.

This patch uses the demandimport.deactivated() context manager.

  Traceback (most recent call last):
    File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 121, in _runcatch
      debugmod = __import__(debugger)
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 115, in _demandimport
      return _hgextimport(_import, name, globals, locals, fromlist, level)
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 47, in _hgextimport
      return importfunc(name, globals, *args)
    File "/usr/lib/python2.7/dist-packages/ipdb/__init__.py", line 16, in <module>
      from ipdb.__main__ import set_trace, post_mortem, pm, run, runcall, runeval, launch_ipdb_on_exception
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 134, in _demandimport
      mod = _hgextimport(_origimport, name, globals, locals)
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 47, in _hgextimport
      return importfunc(name, globals, *args)
    File "/usr/lib/python2.7/dist-packages/ipdb/__main__.py", line 29, in <module>
      if IPython.__version__ > '0.10.2':
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 106, in __getattribute__
      self._load()
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 78, in _load
      mod = _hgextimport(_import, head, globals, locals, None, level)
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 47, in _hgextimport
      return importfunc(name, globals, *args)
    File "/usr/lib/python2.7/dist-packages/IPython/__init__.py", line 45, in <module>
      from .config.loader import Config
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 132, in _demandimport
      return _origimport(name, globals, locals, fromlist, level)
    File "/usr/lib/python2.7/dist-packages/IPython/config/__init__.py", line 16, in <module>
      from .application import *
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 115, in _demandimport
      return _hgextimport(_import, name, globals, locals, fromlist, level)
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 47, in _hgextimport
      return importfunc(name, globals, *args)
    File "/usr/lib/python2.7/dist-packages/IPython/config/application.py", line 30, in <module>
      from IPython.external.decorator import decorator
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 134, in _demandimport
      mod = _hgextimport(_origimport, name, globals, locals)
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 47, in _hgextimport
      return importfunc(name, globals, *args)
    File "/usr/lib/python2.7/dist-packages/IPython/external/decorator/__init__.py", line 2, in <module>
      from decorator import *
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 115, in _demandimport
      return _hgextimport(_import, name, globals, locals, fromlist, level)
    File "/usr/lib/python2.7/dist-packages/mercurial/demandimport.py", line 47, in _hgextimport
      return importfunc(name, globals, *args)
    File "/usr/lib/python2.7/dist-packages/decorator.py", line 240, in <module>
      'ContextManager', (_GeneratorContextManager,), dict(__call__=__call__))

mercurial/dispatch.py

@@ -11,6 +11,7 b' import difflib'
 import util, commands, hg, fancyopts, extensions, hook, error
 import cmdutil, encoding
 import ui as uimod
+import demandimport
 
 class request(object):
     def __init__(self, args, ui=None, repo=None, fin=None, fout=None,
@@ -137,10 +138,11 b' def _runcatch(req):'
                 # This import can be slow for fancy debuggers, so only
                 # do it when absolutely necessary, i.e. when actual
                 # debugging has been requested
-                try:
-                    debugmod = __import__(debugger)
-                except ImportError:
-                    pass # Leave debugmod = pdb
+                with demandimport.deactivated():
+                    try:
+                        debugmod = __import__(debugger)
+                    except ImportError:
+                        pass # Leave debugmod = pdb
 
             debugtrace[debugger] = debugmod.set_trace
             debugmortem[debugger] = debugmod.post_mortem