RefDatabase: add hasRefs convenience method

Callers can now say:

 db.getRefDatabase().hasRefs()

rather than the more verbose:

 !db.getRefDatabase().getAllRefs().isEmpty()

The default implementation simply uses getAllRefs().isEmpty(), but a
derived class could possibly override the method with a more efficient
implementation.

Change-Id: I5244520708a1a7d9adb351f10e43fc39d98e22a1
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/debug/RebuildCommitGraph.java

@@ -115,7 +115,7 @@ class RebuildCommitGraph extends TextBuiltin {
 	/** {@inheritDoc} */
 	@Override
 	protected void run() throws Exception {
-		if (!really && !db.getRefDatabase().getAllRefs().isEmpty()) {
+		if (!really && db.getRefDatabase().hasRefs()) {
 			File directory = db.getDirectory();
 			String absolutePath = directory == null ? "null" //$NON-NLS-1$
 					: directory.getAbsolutePath();

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

@@ -407,6 +407,18 @@ public abstract class RefDatabase {
 		return getRefsByPrefix(ALL);
 	}
 
+	/**
+	 * Check if any refs exist in the ref database.
+	 *
+	 * @return true if the database has refs.
+	 * @throws java.io.IOException
+	 *             the reference space cannot be accessed.
+	 * @since 5.0
+	 */
+	public boolean hasRefs() throws IOException {
+		return !getAllRefs().isEmpty();
+	}
+
 	/**
 	 * Get the additional reference-like entities from the repository.
 	 * <p>