vcs/git: restructure test to avoid crashing ctrlflow analysis

Created by: keegancsmith

golangci-lint is flaky at the moment due to it sometimes crashing on analysing the internal/vcs/git.test package.

This is a stab at the dark for why it is failing. The other likely reason is this package mixes both xtest and test. That is attempted in https://github.com/sourcegraph/sourcegraph/pull/7857