DfsObjDatabase: Add lazy last modified method to PackList

Change-Id: Id045f162fa584ea14da29a9df58a42c53a78dc15

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

@@ -495,10 +495,24 @@ public abstract class DfsObjDatabase extends ObjectDatabase {
 		/** All known packs, sorted. */
 		public final DfsPackFile[] packs;
 
+		private long lastModified = -1;
+
 		PackList(DfsPackFile[] packs) {
 			this.packs = packs;
 		}
 
+		/** @return last modified time of all packs, in milliseconds. */
+		public long getLastModified() {
+			if (lastModified < 0) {
+				long max = 0;
+				for (DfsPackFile pack : packs) {
+					max = Math.max(max, pack.getPackDescription().getLastModified());
+				}
+				lastModified = max;
+			}
+			return lastModified;
+		}
+
 		abstract boolean dirty();
 
 		/**