Obtain IndexCommits from directory

[Jason Rutherglen]

Need to be able to get a list of IndexCommits for a directory.  Also open an
IndexReader for each IndexCommit.  Am thinking of API such as.  I suppose
this could cause problems for reopen.

IndexCommit[] commits = IndexReader.listCommitPoints(Directory directory);

and

IndexReader.open(IndexCommit commit, Directory directory);