PackWriter: Make internal class static

When Error Prone checks are enabled, the "ClassCanBeStatic" warning is
triggered:

  Inner class is non-static but does not reference enclosing class
  see https://errorprone.info/bugpattern/ClassCanBeStatic

Change-Id: I5a0e3bf0cf8c28176d9c98914c1c0dfab9c5736f
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriter.java

@@ -878,7 +878,7 @@ public class PackWriter implements AutoCloseable {
 	 * already been visited at this depth or shallower, it is not necessary to
 	 * re-visit at this depth.
 	 */
-	private class DepthAwareVisitationPolicy
+	private static class DepthAwareVisitationPolicy
 			implements ObjectWalk.VisitationPolicy {
 		private final Map<ObjectId, Integer> lowestDepthVisited = new HashMap<>();