userdiff: PHP: catch "abstract" and "final" functions

PHP permits functions to be defined like

       final public function foo() { }
       abstract protected function bar() { }

but our hunk header pattern does not recognize these decorations.
Add "final" and "abstract" to the list of function modifiers.

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

t/t4018/php-abstract-method

@@ -0,0 +1,7 @@
+abstract class Klass
+{
+    abstract public function RIGHT(): ?string
+    {
+        return 'ChangeMe';
+    }
+}

t/t4018/php-final-method

@@ -0,0 +1,7 @@
+class Klass
+{
+    final public function RIGHT(): string
+    {
+        return 'ChangeMe';
+    }
+}

userdiff.c

@@ -147,7 +147,7 @@ PATTERNS("perl",
 	 "|=~|!~"
 	 "|<<|<>|<=>|>>"),
 PATTERNS("php",
-	 "^[\t ]*(((public|protected|private|static)[\t ]+)*function.*)$\n"
+	 "^[\t ]*(((public|protected|private|static|abstract|final)[\t ]+)*function.*)$\n"
 	 "^[\t ]*((((final|abstract)[\t ]+)?class|interface|trait).*)$",
 	 /* -- */
 	 "[a-zA-Z_][a-zA-Z0-9_]*"