extensions: catch OSError when hgext is not accessible (issue1708)

Temporary workaround for issue1708: on win32 with py2exe, hgext is distributed
inside a zipped file (which anyway does not contain the py files from which we
ought to extract the documentation strings), which raises a WindowsError
(subclasses OSError).

This means that on such platforms the list of disabled extensions won't be
available. Real fix postponed for after Mercurial 1.3.

mercurial/extensions.py

@@ -125,9 +125,14 b' def disabled():'
     import hgext
     extpath = os.path.dirname(os.path.abspath(hgext.__file__))
 
+    try: # might not be a filesystem path
+        files = os.listdir(extpath)
+    except OSError:
+        return None, 0
+
     exts = {}
     maxlength = 0
-    for e in os.listdir(extpath):
+    for e in files:
 
         if e.endswith('.py'):
             name = e.rsplit('.', 1)[0]