Contributed by Steve Loughran.
This patch
* changes the default for the staging committer to append, as we get for the classic FileOutputFormat committer
* adds a check for the dest path being a file not a dir
* adds tests for this
* Changes AbstractCommitTerasortIT. to not use the simple parser, so fails if the file is present.
Change-Id: Id53742958ed1cf321ff96c9063505d64f3254f53