Expose LFS operation strings as public constants

So that they can be used either by third party LFS server implementations
or from within other parts of JGit's internal LFS implementation.

Change-Id: I58da6230247204588e017c010ce5b14e4615448d
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.lfs.server/src/org/eclipse/jgit/lfs/server/TransferHandler.java

@@ -44,6 +44,9 @@
 package org.eclipse.jgit.lfs.server;
 
 import static javax.servlet.http.HttpServletResponse.SC_NOT_FOUND;
+import static org.eclipse.jgit.lfs.lib.Constants.DOWNLOAD;
+import static org.eclipse.jgit.lfs.lib.Constants.UPLOAD;
+import static org.eclipse.jgit.lfs.lib.Constants.VERIFY;
 
 import java.io.IOException;
 import java.text.MessageFormat;
@@ -58,18 +61,14 @@ import org.eclipse.jgit.lfs.server.internal.LfsServerText;
 
 abstract class TransferHandler {
 
-	private static final String DOWNLOAD = "download"; //$NON-NLS-1$
-	private static final String UPLOAD = "upload"; //$NON-NLS-1$
-	private static final String VERIFY = "verify"; //$NON-NLS-1$
-
 	static TransferHandler forOperation(String operation,
 			LargeFileRepository repository, List<LfsObject> objects) {
 		switch (operation) {
-		case TransferHandler.UPLOAD:
+		case UPLOAD:
 			return new Upload(repository, objects);
-		case TransferHandler.DOWNLOAD:
+		case DOWNLOAD:
 			return new Download(repository, objects);
-		case TransferHandler.VERIFY:
+		case VERIFY:
 		default:
 			throw new UnsupportedOperationException(MessageFormat.format(
 					LfsServerText.get().unsupportedOperation, operation));

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

@@ -86,6 +86,27 @@ public final class Constants {
 	public static final int LONG_OBJECT_ID_STRING_LENGTH = LONG_OBJECT_ID_LENGTH
 			* 2;
 
+	/**
+	 * LFS upload operation.
+	 *
+	 * @since 4.7
+	 */
+	public static final String UPLOAD = "upload";
+
+	/**
+	 * LFS download operation.
+	 *
+	 * @since 4.7
+	 */
+	public static final String DOWNLOAD = "download";
+
+	/**
+	 * LFS verify operation.
+	 *
+	 * @since 4.7
+	 */
+	public static final String VERIFY = "verify";
+
 	/**
 	 * Create a new digest function for objects.
 	 *