userdiff: improve java hunk header regex

Currently, the git diff hunk headers show the wrong method signature if the
method has a qualified return type, an array return type, or a generic return
type because the regex doesn't allow dots (.), [], or < and > in the return
type.  Also, type parameter declarations couldn't be matched.

Add several t4018 tests asserting the right hunk headers for different cases:

  - enum constant change
  - change in generic method with bounded type parameters
  - change in generic method with wildcard
  - field change in a nested class

Signed-off-by: Tassilo Horn <tsdh@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4018/java-enum-constant

@@ -0,0 +1,6 @@
+private enum RIGHT {
+    ONE,
+    TWO,
+    THREE,
+    ChangeMe
+}