Support multiple source repositories

[smarr]

Make it possible to track info of multiple source repositories.

For instance, for the TruffleRuby project, benchmarks and language implementation my be in different repositories.
So, it would be necessary to track the info of both repositories to get a complete picture.

There are other use cases, for instance the Are We Fast Yet project, but since we currently use submodules there, it hasn't been as much of an issue yet.

For TruffleRuby, the current solution is to check out the benchmarks into a subdirectory, and delete its .git folder so that rebench picks up the outer folder's git info.
But this means, we don't track the version details of the benchmarks.