userdiff-cpp: learn the C++ spaceship operator

Since C++20, the language has a generalized comparison operator <=>.
Teach the cpp driver not to separate it into <= and > tokens.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4034/cpp/expect

@@ -25,7 +25,7 @@ str.e+<RED>65<RESET><GREEN>75<RESET>
 a<RED>*<RESET><GREEN>*=<RESET>b c<RED>/<RESET><GREEN>/=<RESET>d e<RED>%<RESET><GREEN>%=<RESET>f
 a<RED>+<RESET><GREEN>++<RESET>b c<RED>-<RESET><GREEN>--<RESET>d
 a<RED><<<RESET><GREEN><<=<RESET>b c<RED>>><RESET><GREEN>>>=<RESET>d
-a<RED><<RESET><GREEN><=<RESET>b c<RED><=<RESET><GREEN><<RESET>d e<RED>><RESET><GREEN>>=<RESET>f g<RED>>=<RESET><GREEN>><RESET>h i<=<GREEN>><RESET>j
+a<RED><<RESET><GREEN><=<RESET>b c<RED><=<RESET><GREEN><<RESET>d e<RED>><RESET><GREEN>>=<RESET>f g<RED>>=<RESET><GREEN>><RESET>h i<RED><=<RESET><GREEN><=><RESET>j
 a<RED>==<RESET><GREEN>!=<RESET>b c<RED>!=<RESET><GREEN>=<RESET>d
 a<RED>^<RESET><GREEN>^=<RESET>b c<RED>|<RESET><GREEN>|=<RESET>d e<RED>&&<RESET><GREEN>&=<RESET>f
 a<RED>||<RESET><GREEN>|<RESET>b

userdiff.c

@@ -72,7 +72,7 @@ PATTERNS("cpp",
 	 "|0[xXbB][0-9a-fA-F']+[lLuU]*"
 	 /* floatingpoint numbers that begin with a decimal point */
 	 "|\\.[0-9][0-9']*([Ee][-+]?[0-9]+)?[fFlL]?"
-	 "|[-+*/<>%&^|=!]=|--|\\+\\+|<<=?|>>=?|&&|\\|\\||::|->\\*?|\\.\\*"),
+	 "|[-+*/<>%&^|=!]=|--|\\+\\+|<<=?|>>=?|&&|\\|\\||::|->\\*?|\\.\\*|<=>"),
 PATTERNS("csharp",
 	 /* Keywords */
 	 "!^[ \t]*(do|while|for|if|else|instanceof|new|return|switch|case|throw|catch|using)\n"