Switch to shelling out to `git`

[joelanford]

There have been several issues and PRs that make me question go-apidiff's dependency on github.com/go-git/go-git:

• git operation not respecting gitignore #20
• pkg/diff: respect global .gitignore #25
• improve error message for dirty git tree #12
• get packages: workaround various go-git checkout issues #8
• fix: handle repositories with symlinks targeting absolute paths #5

Perhaps it makes more sense to:

1. ensure that a git executable is available as a precondition
2. shell out to git rather than use github.com/go-git/go-git