Show existing data when opening the site locally

[Kobzol]

When you run the site locally, and open the compare.html page (which is shown by default), you will be greated with a very annoying error message (which essentially tells you that the backend has panicked, or something like that). It would be great if the default view for compare.html examined the database, and tried to show some reasonable data by default, e.g. the latest entry found in the DB and -30 days before that (same as what it does on the production web).

If there is only a single commit, it should be compared with itself (this can be useful to see absolute numbers of the benchmark metrics), to again avoid showing the error message.