Clean up the mining code

[vovak]

Currently our mining tool is far from nice: the implementation is obscure, undocumented, and hardcoded for the IntelliJ repo in places.

Required to make #10 and #11 possible:

• Remove hardcoded fragments

Also (better be done by the FSE deadline as we are going to share the code for reproducibility):

• Refactor
• Add comments
• Switch to jgit and get rid of the Python backend