userdiff: better method/property matching for C#

- Support multi-line methods by not requiring closing parenthesis.
- Support multiple generics (comma was missing before).
- Add missing `foreach`, `lock` and  `fixed` keywords to skip over.
- Remove `instanceof` keyword, which isn't C#.
- Also detect non-method keywords not positioned at the start of a line.
- Added tests; none existed before.

The overall strategy is to focus more on what isn't expected for
method/property definitions, instead of what is, but is fully optional.

Signed-off-by: Steven Jeuris <steven.jeuris@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/csharp-exclude-control-statements

@@ -0,0 +1,34 @@
+class Example
+{
+    string Method(int RIGHT)
+    {
+        if (false)
+        {
+            return "out";
+        }
+        else { }
+        if (true) MethodCall(
+        );
+        else MethodCall(
+        );
+        switch ("test")
+        {
+            case "one":
+            return MethodCall(
+            );
+            case "two":
+            break;
+        }
+        (int, int) tuple = (1, 4);
+        switch (tuple)
+        {
+            case (1, 4):
+              MethodCall();
+		      break;
+        }
+
+        return "ChangeMe";
+    }
+
+    string MethodCall(int a = 0, int b = 0) => "test";
+}