gitaly: Become gitconfig-clean
This MR makes the Go-side of Gitaly gitconfig-clean. With this change none of our tests pick up the gitconfig anymore.
Note that this is not in fact verified as part of this MR. Instead, there's !4576 (closed) which will start to verify that, but that MR also depends on !4577 (merged).
Part of #4242 (closed)
Edited by Patrick Steinhardt