Fix BadComparable error flagged by error prone

Running recent error prone version complaining on that code:

LfsPointer.java:171: error: [BadComparable] Possible sign flip from
narrowing conversion
		return (int) (getSize() - o.getSize());
		       ^
    (see https://errorprone.info/bugpattern/BadComparable)
  Did you mean 'return Long.compare(getSize(), o.getSize());'?

Bug: 562756
Change-Id: I0522f1025319a9290c448a064fbafdb4b16d1d59
Signed-off-by: David Ostrovsky <david@ostrovsky.org>

org.eclipse.jgit.lfs/src/org/eclipse/jgit/lfs/LfsPointer.java

@@ -168,7 +168,7 @@ public class LfsPointer implements Comparable<LfsPointer> {
 			return x;
 		}
 
-		return (int) (getSize() - o.getSize());
+		return Long.compare(getSize(), o.getSize());
 	}
 }