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-class-member-function

@@ -3,6 +3,10 @@ public class Beer
 	int special;
 	public static void main(String RIGHT[])
 	{
+		someMethodCall();
+		someOtherMethod("17")
+			.doThat();
+		// Whatever
 		System.out.print("ChangeMe");
 	}
 }

t/t4018/java-enum-constant

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

t/t4018/java-method-return-generic-bounded

@@ -0,0 +1,9 @@
+class MyExample {
+    public <T extends Bar & Foo<T>, R> Map<T, R[]> foo(String[] RIGHT) {
+        someMethodCall();
+        someOtherMethod()
+            .doThat();
+        // Whatever...
+        return (List<T>) Arrays.asList("ChangeMe");
+    }
+}

t/t4018/java-method-return-generic-wildcard

@@ -0,0 +1,9 @@
+class MyExample {
+    public List<? extends Comparable> foo(String[] RIGHT) {
+        someMethodCall();
+        someOtherMethod()
+            .doThat();
+        // Whatever...
+        return Arrays.asList("ChangeMe");
+    }
+}

t/t4018/java-nested-field

@@ -0,0 +1,6 @@
+class MyExample {
+    private static class RIGHT {
+        // change an inner class field
+        String inner = "ChangeMe";
+    }
+}

userdiff.c

@@ -142,7 +142,11 @@ PATTERNS("html",
 	 "[^<>= \t]+"),
 PATTERNS("java",
 	 "!^[ \t]*(catch|do|for|if|instanceof|new|return|switch|throw|while)\n"
-	 "^[ \t]*(([A-Za-z_][A-Za-z_0-9]*[ \t]+)+[A-Za-z_][A-Za-z_0-9]*[ \t]*\\([^;]*)$",
+	 /* Class, enum, and interface declarations */
+	 "^[ \t]*(([a-z]+[ \t]+)*(class|enum|interface)[ \t]+[A-Za-z][A-Za-z0-9_$]*[ \t]+.*)$\n"
+	 /* Method definitions; note that constructor signatures are not */
+	 /* matched because they are indistinguishable from method calls. */
+	 "^[ \t]*(([A-Za-z_<>&][][?&<>.,A-Za-z_0-9]*[ \t]+)+[A-Za-z_][A-Za-z_0-9]*[ \t]*\\([^;]*)$",
 	 /* -- */
 	 "[a-zA-Z_][a-zA-Z0-9_]*"
 	 "|[-+0-9.e]+[fFlL]?|0[xXbB]?[0-9a-fA-F]+[lL]?"