Add getPacks to ObjectDirectory

This exposes the list of known packs, allowing callers to list them
into a context like the objects/info/packs file.

Change-Id: I0b889564bd176836ff5c77ba310c6d229409dcd5
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

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

@@ -147,6 +147,18 @@ private File fileFor(final String objectName) {
 		return new File(new File(objects, d), f);
 	}
 
+	/**
+	 * @return unmodifiable collection of all known pack files local to this
+	 *         directory. Most recent packs are presented first. Packs most
+	 *         likely to contain more recent objects appear before packs
+	 *         containing objects referenced by commits further back in the
+	 *         history of the repository.
+	 */
+	public Collection<PackFile> getPacks() {
+		final PackFile[] packs = packList.get().packs;
+		return Collections.unmodifiableCollection(Arrays.asList(packs));
+	}
+
 	/**
 	 * Add a single existing pack to the list of available pack files.
 	 *