Git v2.38.0 brings some important changes with it that we want to build
on:

    - It fixes a source of corruption when upgrading commit-graphs that
      contain generation data in v1 to v2. We currently had to disable
      use of generation data completely because of that bug so that we
      don't corrupt repositories. This information is important though
      to speed up some computations, like for example in the packfile
      negotiation.

    - git-merge-tree(1) learned two merge two trees. This is important
      to allow us to do in-memory merges.

    - git-cat-file(1) starts to support mailmaps, which is important for
      us to support them in Gitaly.

    - git-clone(1) has clearned about bundle URIs, which allows
      offloading parts of a clone to a CDN.

Bump the minimum required Git version to v2.38.0.

Changelog: changed