dfs: only create DfsPackFile if description has PACK

In the future with reftable a DFS implementation may choose to create
a PackDescription that contains only a REFTABLE extension.  Filter
these out by only creating a DfsPackFile if the PackDescription as the
expected PackExt.PACK.

Change-Id: I4c831622378156ae6b68f82c1ee1db5e150893be

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsObjDatabase.java

@@ -464,7 +464,7 @@ public abstract class DfsObjDatabase extends ObjectDatabase {
 			DfsPackFile oldPack = forReuse.remove(dsc);
 			if (oldPack != null) {
 				list.add(oldPack);
-			} else {
+			} else if (dsc.hasFileExt(PackExt.PACK)) {
 				list.add(new DfsPackFile(cache, dsc));
 				foundNew = true;
 			}
@@ -482,8 +482,7 @@ public abstract class DfsObjDatabase extends ObjectDatabase {
 	}
 
 	private static Map<DfsPackDescription, DfsPackFile> reuseMap(PackList old) {
-		Map<DfsPackDescription, DfsPackFile> forReuse
-			= new HashMap<>();
+		Map<DfsPackDescription, DfsPackFile> forReuse = new HashMap<>();
 		for (DfsPackFile p : old.packs) {
 			if (p.invalid()) {
 				// The pack instance is corrupted, and cannot be safely used