Fix DirCache.isModified()

Change I61a1b45db2d60fdcc0f87373ac6fd75ac4c4a202 fixed a possible NPE
occurring for newly created repositories - but in that case a wrong
value (false = not modified) was returned.

If a current version of the index file exists (liveFile), but there is
no snapshot, this means that there have been modifications (i.e. true
has to be returned).

Change-Id: I698f78112249f9924860fc58eb7eab7afdf87eb7
Signed-off-by: Philipp Thun <philipp.thun@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCache.java

@@ -321,7 +321,7 @@ public class DirCache {
 	public boolean isOutdated() throws IOException {
 		if (liveFile == null || !liveFile.exists())
 			return false;
-		return snapshot != null && snapshot.isModified(liveFile);
+		return snapshot == null || snapshot.isModified(liveFile);
 	}
 
 	/** Empty this index, removing all entries. */