ui: add brief comment why raw_input() needs dummy ' ' prompt string

Retrieved from 17ffb30d9174.

I was about to move ' ' to label(msg + ' ', 'ui.prompt') so that subclass can
distinguish prompt output from data.  But it was not that simple.

mercurial/ui.py

@@ -626,6 +626,8 b' class ui(object):'
         oldout = sys.stdout
         sys.stdin = self.fin
         sys.stdout = self.fout
+        # prompt ' ' must exist; otherwise readline may delete entire line
+        # - http://bugs.python.org/issue12833
         line = raw_input(' ')
         sys.stdin = oldin
         sys.stdout = oldout