Fix lastModified to be consistent in DfsGarbageCollector

Set all packs written by the DfsGarbageCollector to use the same
starting timestamp as lastModified. This makes it easier to see
which packs came from the same DfsGarbageCollector run, as they
share the same timestamp.

Change-Id: Id633573fbc3f0f360887b4745cacf33d6fc09320

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

@@ -95,8 +95,8 @@ public class DfsGarbageCollector {
 
 	private long coalesceGarbageLimit = 50 << 20;
 
+	private long startTimeMillis;
 	private List<DfsPackFile> packsBefore;
-
 	private Set<ObjectId> allHeads;
 	private Set<ObjectId> nonHeads;
 	private Set<ObjectId> txnHeads;
@@ -190,6 +190,7 @@ public class DfsGarbageCollector {
 			throw new IllegalStateException(
 					JGitText.get().supportOnlyPackIndexVersion2);
 
+		startTimeMillis = System.currentTimeMillis();
 		ctx = (DfsReader) objdb.newReader();
 		try {
 			refdb.refresh();
@@ -422,6 +423,7 @@ public class DfsGarbageCollector {
 
 		PackStatistics stats = pw.getStatistics();
 		pack.setPackStats(stats);
+		pack.setLastModified(startTimeMillis);
 		newPackStats.add(stats);
 		newPackObj.add(pw.getObjectSet());