hook: disable demandimport before importing hooks

This solved an obscure bug for me. In upgrading Distribute on the server, a
patch was added to has a try: import a except ImportError thing that's only
supposed to work with Python 3.3. I'm using 2.7. My hook failed with an
ImportError because of this. It seems kind of sensible to turn off
demandimport before importing the hook, since the except ImportError pattern
is used quite a bit in Python code (including in other Distribute code).

mercurial/hook.py

@@ -7,7 +7,7 b''
 
 from i18n import _
 import os, sys
-import extensions, util
+import extensions, util, demandimport
 
 def _pythonhook(ui, repo, name, hname, funcname, args, throw):
     '''call python hook. hook is callable object, looked up as
@@ -35,13 +35,17 b' def _pythonhook(ui, repo, name, hname, f'
                 sys.path = sys.path[:] + [modpath]
                 modname = modfile
         try:
+            demandimport.disable()
             obj = __import__(modname)
+            demandimport.enable()
         except ImportError:
             e1 = sys.exc_type, sys.exc_value, sys.exc_traceback
             try:
                 # extensions are loaded with hgext_ prefix
                 obj = __import__("hgext_%s" % modname)
+                demandimport.enable()
             except ImportError:
+                demandimport.enable()
                 e2 = sys.exc_type, sys.exc_value, sys.exc_traceback
                 if ui.tracebackflag:
                     ui.warn(_('exception from first failed import attempt:\n'))