gomod: Replace deprecated src-d/go-git
Created by: keegancsmith
Both gopkg.in/src-d/go-git.v4 and github.com/go-git/go-git/v5 are imported in our project, and src-d/go-git is already deprecated. Let's switch to go-git/go-git.
Signed-off-by: blanet [email protected]
Merging of forks is currently broken it seems. So this is a recreation of https://github.com/sourcegraph/sourcegraph/pull/33769 by myself.
Test Plan: CI