Replace custom encoder `Constants#encode` by JDK implementation

Using the implementation provided in the JDK since Java 1.6 by
`String#getBytes(Charset)` reduces JGit maintenance effort and improves
performance.

The method Constants#encode was implemented when JGit still used Java
1.5. See [1].

Kudos to Marcin for proposing to use this improvement in RefWriter [2].
I think it should be used generally.

[1] https://repo.or.cz/jgit.git?a=commit;h=bfa3da225f198b19061158499b1135aff07d85b3
[2] https://eclipse.gerrithub.io/c/eclipse-jgit/jgit/+/1195180

Also-By: Marcin Czech <maczech@gmail.com>
Change-Id: I361ed6286b98351a315b8a8ffc3cb845831d35b2

org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java
index 3eb35be..c27047f 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java
@@ -14,7 +14,6 @@
 
 import static java.nio.charset.StandardCharsets.UTF_8;
 
-import java.nio.ByteBuffer;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.text.MessageFormat;
@@ -734,17 +733,7 @@ public static int decodeTypeString(final AnyObjectId id,
 	 *         default character encoding (UTF-8).
 	 */
 	public static byte[] encode(String str) {
-		final ByteBuffer bb = UTF_8.encode(str);
-		final int len = bb.limit();
-		if (bb.hasArray() && bb.arrayOffset() == 0) {
-			final byte[] arr = bb.array();
-			if (arr.length == len)
-				return arr;
-		}
-
-		final byte[] arr = new byte[len];
-		bb.get(arr);
-		return arr;
+		return str.getBytes(UTF_8);
 	}
 
 	static {