color: make a test for curse availability explicite

We won't use terminfo when curse failed to load. Before this change, we were
doing an indirect test, relying on the fact some variable ('_terminfo_params')
would be empty if curses failed to load. We update the code to be more explicit
and directly checks if we managed to load the curse module.

hgext/color.py

@@ -196,7 +196,7 b' def _terminfosetup(ui, mode):'
     '''Initialize terminfo data and the terminal if we're in terminfo mode.'''
 
     # If we failed to load curses, we go ahead and return.
-    if not color._terminfo_params:
+    if curses is None:
         return
     # Otherwise, see what the config file says.
     if mode not in ('auto', 'terminfo'):