crecord: avoid setting non-existing SIGTSTP signal on windows (issue5512)

Windows do not have a SIGTSTP so we avoid setting the handler if the signal is
unknown.

mercurial/crecord.py

@@ -473,14 +473,17 b' def chunkselector(ui, headerlist, operat'
     """
     ui.write(_('starting interactive selection\n'))
     chunkselector = curseschunkselector(headerlist, ui, operation)
-    f = signal.getsignal(signal.SIGTSTP)
+    origsigtstp = sentinel = object()
+    if util.safehasattr(signal, 'SIGTSTP'):
+        origsigtstp = signal.getsignal(signal.SIGTSTP)
     try:
         curses.wrapper(chunkselector.main)
         if chunkselector.initerr is not None:
             raise error.Abort(chunkselector.initerr)
         # ncurses does not restore signal handler for SIGTSTP
     finally:
-        signal.signal(signal.SIGTSTP, f)
+        if origsigtstp is not sentinel:
+            signal.signal(signal.SIGTSTP, origsigtstp)
     return chunkselector.opts
 
 def testdecorator(testfn, f):