PacketLineIn: Deprecate the END constant

Deprecate the constant with the intention of making it private in
a future release.

All existing usage of the constant within JGit code has already been
replaced with the recommended alternatives in preceding commits.

Change-Id: I10eb95f3f92cb74f93a26bf1a6edd24615b75c6f
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineIn.java

@@ -74,7 +74,15 @@ import org.slf4j.LoggerFactory;
 public class PacketLineIn {
 	private static final Logger log = LoggerFactory.getLogger(PacketLineIn.class);
 
-	/** Magic return from {@link #readString()} when a flush packet is found. */
+	/**
+	 * Magic return from {@link #readString()} when a flush packet is found.
+	 *
+	 * @deprecated Callers should use {@link #isEnd(String)} to check if a
+	 *             string is the end marker, or
+	 *             {@link PacketLineIn#readStrings()} to iterate over all
+	 *             strings in the input stream until the marker is reached.
+	 */
+	@Deprecated
 	public static final String END = new StringBuilder(0).toString(); 	/* must not string pool */
 
 	/**