MAPREDUCE-5530. Fix compat with hadoop-1 in mapred.lib.CombinFileInputFormat by re-introducing isSplittable(FileSystem, Path) api and ensuring semantic compatibility. Contributed by Robert Kanter.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1528533 13f79535-47bb-0310-9956-ffa450edef68