Speed up check for modifications of tracked resources

We only need to check file existense if some other stat returns
a value that may mean that the file does not exist. File.length() == 0
or File.lastModified() == 0 are two such properties. We use length
here.

Change-Id: If626b12e7bb4da994b5c086f6a5b7a12c187261c
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>

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

 				return true;
 
 			File file = getFile(wd);
-			if (!file.exists())
+			long length = file.length();
+			if (length == 0) {
+				if (!file.exists())
+					return true;
+			}
+			if (length != size)
 				return true;
 
 			// JDK1.6 has file.canExecute
 				}
 			}
 
-			if (file.length() != size)
-				return true;
-
 			// Git under windows only stores seconds so we round the timestamp
 			// Java gives us if it looks like the timestamp in index is seconds
 			// only. Otherwise we compare the timestamp at millisecond prevision.