userdiff: recognize 'macro_rules!' as starting a Rust function block

Signed-off-by: Konrad Borowski <konrad@borowski.pw>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/rust-macro-rules

@@ -0,0 +1,6 @@
+macro_rules! RIGHT {
+    () => {
+        // a comment
+        let x = ChangeMe;
+    };
+}

userdiff.c

@@ -165,7 +165,7 @@ PATTERNS("ruby", "^[ \t]*((class|module|def)[ \t].*)$",
 	 "|[-+0-9.e]+|0[xXbB]?[0-9a-fA-F]+|\\?(\\\\C-)?(\\\\M-)?."
 	 "|//=?|[-+*/<>%&^|=!]=|<<=?|>>=?|===|\\.{1,3}|::|[!=]~"),
 PATTERNS("rust",
-	 "^[\t ]*((pub(\\([^\\)]+\\))?[\t ]+)?((async|const|unsafe|extern([\t ]+\"[^\"]+\"))[\t ]+)?(struct|enum|union|mod|trait|fn|impl)[< \t]+[^;]*)$",
+	 "^[\t ]*((pub(\\([^\\)]+\\))?[\t ]+)?((async|const|unsafe|extern([\t ]+\"[^\"]+\"))[\t ]+)?(struct|enum|union|mod|trait|fn|impl|macro_rules!)[< \t]+[^;]*)$",
 	 /* -- */
 	 "[a-zA-Z_][a-zA-Z0-9_]*"
 	 "|[0-9][0-9_a-fA-Fiosuxz]*(\\.([0-9]*[eE][+-]?)?[0-9_fF]*)?"