Extend information about `branches` field in the `git` index
It is important to add more information about this field. Our customers must know:
- this field is empty by default and it won't be populated until they want us to enable this extra enrichment
- it is very heavy in terms of resources needed, it may require many hours and rewrite all the information about commits in the index
- while it is running no new commits are collected from the data source
- some commits will have no branch: we need a well explained description of this scenario