git/stats: Refactor code parsing git-count-objects(1) to use struct

The stats package contains code that parses git-count-objects(1)'s output into an unstructured map. This map is then used to provide multiple different bits of information like the object count or gets used to write a log message.

This information is more generally useful though. One usecase is to precompute the current repository's shape in OptimizeRepository() and pass it around for use by our heuristics that determine whether things need repacking or not. There is also additional information that's not provided by Git, like whether repositories have bitmaps or not, that we could easily compute here.

But the current design where we parse information into an unstructured map is holding us back: it is not discoverable, makes refactorings unsafe, and is simply not a great interface to use more generally.

Refactor the code into a new ObjectsInfo structure instead to change this. While it requires more intimacy with git-count-objects(1), this seems like an acceptable tradeoff to make wider use of this interface in the future.

Part of #4543 (closed).