Makefile: Upgrade Git to v2.37.4.gl1 and v2.38.2.gl1 respectively

Git has recently created the bugfix release v2.38.2. This commit upgrades the Git version used by Gitaly to pull in Git v2.38.2.gl1, which contains all bugfixes of Git v2.38.2. Furthermore, we have backported a bunch of patches to fix performance issues when pushing into repostiories with loads of references. These backported changes have been released as part of Git v2.39.0 already.

While at it, also bump the Git v2.37 release series to use v2.37.4.gl1. This includes bugfixes as well as fixes for CVEs that don't apply in the context of Gitaly. Note that the naming of the GIT_VERSION_2_37_1 variable is now kind of awkward as it refers to Git v2.37.4.gl1. We're phasing out this Git version anyway though, so it doesn't feel worth it to adjust naming of this variable now.

Changelog: fixed