Extract constants for reflog entry message prefixes

Document explicitly that these are untranslated to (mostly) match C git.

Change-Id: I3abcffb4fd611d053bf4373e5d6a14a66f7b9b6b

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectoryUpdate.java

@@ -50,6 +50,7 @@ import java.io.IOException;
 
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.RefUpdate;
+import org.eclipse.jgit.lib.ReflogEntry;
 import org.eclipse.jgit.lib.Repository;
 
 /** Updates any reference stored by {@link RefDirectory}. */
@@ -127,14 +128,14 @@ class RefDirectoryUpdate extends RefUpdate {
 		return status;
 	}
 
-	private String toResultString(final Result status) {
+	private String toResultString(Result status) {
 		switch (status) {
 		case FORCED:
-			return "forced-update"; //$NON-NLS-1$
+			return ReflogEntry.PREFIX_FORCED_UPDATE;
 		case FAST_FORWARD:
-			return "fast forward"; //$NON-NLS-1$
+			return ReflogEntry.PREFIX_FAST_FORWARD;
 		case NEW:
-			return "created"; //$NON-NLS-1$
+			return ReflogEntry.PREFIX_CREATED;
 		default:
 			return null;
 		}

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

@@ -49,6 +49,39 @@ package org.eclipse.jgit.lib;
  */
 public interface ReflogEntry {
 
+	/**
+	 * Prefix used in reflog messages when the ref was first created.
+	 * <p>
+	 * Does not have a corresponding constant in C git, but is untranslated like
+	 * the other constants.
+	 *
+	 * @since 4.9
+	 */
+	public static final String PREFIX_CREATED = "created"; //$NON-NLS-1$
+
+	/**
+	 * Prefix used in reflog messages when the ref was updated with a fast
+	 * forward.
+	 * <p>
+	 * Untranslated, and exactly matches the
+	 * <a href="https://git.kernel.org/pub/scm/git/git.git/tree/builtin/fetch.c?id=f3da2b79be9565779e4f76dc5812c68e156afdf0#n680">
+	 * untranslated string in C git</a>.
+	 *
+	 * @since 4.9
+	 */
+	public static final String PREFIX_FAST_FORWARD = "fast-forward"; //$NON-NLS-1$
+
+	/**
+	 * Prefix used in reflog messages when the ref was force updated.
+	 * <p>
+	 * Untranslated, and exactly matches the
+	 * <a href="https://git.kernel.org/pub/scm/git/git.git/tree/builtin/fetch.c?id=f3da2b79be9565779e4f76dc5812c68e156afdf0#n695">
+	 * untranslated string in C git</a>.
+	 *
+	 * @since 4.9
+	 */
+	public static final String PREFIX_FORCED_UPDATE = "forced-update"; //$NON-NLS-1$
+
 	/**
 	 * @return the commit id before the change
 	 */