vcs/git: Sandboxed .gitconfig in all git commands
Created by: tsenart
Background
Tests were failing for me because of some special .gitconfig options I have set. We should not be reading or messing with system level or the $USER's .gitconfig, only with the repo scoped one.
This commit makes it so that all git commands are ran with the repo's
local .git/config
file.
Notes
Extracted from #1391.