Use the new FS.exists method in commonly occuring places

Allegedly this should improve performance, but I could not see it.

Change-Id: Id2057cb2cfcb46e94ff954483ce23f9c4a7edc5e

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java

 
 import java.io.File;
 
+import org.eclipse.jgit.util.FS;
+
 /**
  * Caches when a file was last read, making it possible to detect future edits.
  * <p>
 	public static final FileSnapshot MISSING_FILE = new FileSnapshot(0, 0) {
 		@Override
 		public boolean isModified(File path) {
-			return path.exists();
+			return FS.DETECTED.exists(path);
 		}
 	};
 

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/ObjectDirectory.java

 
 	@Override
 	public boolean exists() {
-		return objects.exists();
+		return fs.exists(objects);
 	}
 
 	@Override
 		}
 
 		final File dst = fileFor(id);
-		if (dst.exists()) {
+		if (fs.exists(dst)) {
 			// We want to be extra careful and avoid replacing an object
 			// that already exists. We can't be sure renameTo() would
 			// fail on all platforms if dst exists, so we check first.

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectory.java

 			for (String refName : refs) {
 				// Lock the loose ref
 				File refFile = fileFor(refName);
-				if (!refFile.exists())
+				if (!fs.exists(refFile))
 					continue;
 				LockFile rLck = new LockFile(refFile,
 						parent.getFS());

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/FileTreeIterator.java

 			file = f;
 
 			if (f.isDirectory()) {
-				if (new File(f, Constants.DOT_GIT).exists())
+				if (fs.exists(new File(f, Constants.DOT_GIT)))
 					mode = FileMode.GITLINK;
 				else
 					mode = FileMode.TREE;

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/WorkingTreeIterator.java

 
 		private void loadRulesFromFile(IgnoreNode r, File exclude)
 				throws FileNotFoundException, IOException {
-			if (exclude.exists()) {
+			if (FS.DETECTED.exists(exclude)) {
 				FileInputStream in = new FileInputStream(exclude);
 				try {
 					r.parse(in);