store latest revision for file based history cache

[vladak]

When debugging fix for issue #728 I discovered (with quite a shock) that last indexed changeset for file based history is not stored (FileHistoryCache.java):

371    @Override
372    public String getLatestCachedRevision(Repository repository) {
373        return null;
374    }

and therefore full history reindex is done every time.

It should be possible to stash the latest indexed changeset for each project somewhere under the data dir.

[vladak]

This should also help @xsumbe00 with his huge Mercurial repository where hg log for the top-level directory takes many hours to complete.

[vladak]

I am marking this as a stopper since for our internal deployment it is not sure if we can get JavaDB to scale (due to the renamed handling changes done in #13).