Merged revision(s) 1441726 from lucene/dev/trunk:
LUCENE-4740: Don't track clones of MMapIndexInput if unmapping is disabled. This reduces GC overhead.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1441727 13f79535-47bb-0310-9956-ffa450edef68