histedit: conditionalize the imports of 'fcntl' and 'termios'

The recent import of chistedit in c36175456350 made Windows sad.  I'm not sure
if there's other stuff that needs to be done here (e.g. change the default
interface), but this makes the tests run again.

It would have been nicer if the error message indicated these modules were the
problem, but instead it said "*** failed to import extension histedit: No module
named histedit".  I'm not sure if there's anything we can do about that.

hgext/histedit.py

@@ -183,11 +183,17 unexpectedly::
 
 from __future__ import absolute_import
 
-import fcntl
+# chistedit dependencies that are not available everywhere
+try:
+    import fcntl
+    import termios
+except ImportError:
+    fcntl = None
+    termios = None
+
 import functools
 import os
 import struct
-import termios
 
 from mercurial.i18n import _
 from mercurial import (