Fix for UTF-8 roundtrip verification bug.

The problem was that the round-trip verification code copies the
current state, which may contain intermediate Parser state for
multibyte characters and ANSI escape sequences.  It then applies diffs
to that copy, which may appear as badly formed input.

Also removes some dead, never-used code.

src/statesync/completeterminal.h

@@ -67,8 +67,7 @@ namespace Terminal {
     std::string act( const Parser::Action *act );
 
     const Framebuffer & get_fb( void ) const { return terminal.get_fb(); }
-    bool parser_grounded( void ) const { return parser.is_grounded(); }
-
+    void reset_input( void ) { parser.reset_input(); }
     uint64_t get_echo_ack( void ) const { return echo_ack; }
     bool set_echo_ack( uint64_t now );
     void register_input_frame( uint64_t n, uint64_t now );