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git/stats: Expose commit-graph information on generation data

Patrick Steinhardt requested to merge pks-git-stats-commit-graph-generation-data into master

Commit-graphs optionally carry GDA2 and GCO2 information when Git is asked to generate them via commitGraph.generationNumber=2. While this kind of information should've theoretically already exist in the form of the GDAT and GCOV chunks, due to a bug in Git v2.36.0 and earlier Git actually always ignored this kind of data.

Due to a bug that caused corruption of commit-graphs that was introduced when upgrading from v1 to v2 we disabled computing generation numbers altogether. Generation numbers are an important optimization in some contexts though, so we should aim to eventually reenable generation of them.

As a first step towards this let's start exposing information about whether commit-graphs have generation data or not via the git/stats package. Note that we only check for the v2 chunks GDA2 and GCO2 as the other formats weren't actually ever consumed in the first place. And current Git versions wouldn't know to write these anyway.

Changelog: added

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