Add git 2.38 as a build target in Makefile
In order to start using git 2.38, we need to add it as a new target in the Makefile.
This is the first step to upgrading the Git version in bundled git as outlined in https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#git-version-upgrades
part of #4509 (closed)
Edited by John Cai