Use new go-git functionality to fetch commits

[tianon]

This avoids shelling out by using the implementation from go-git/go-git#58 / go-git/go-git@8ecd388, which is going to be much more performant.

[tianon]

Proof's in the pudding -- here's fetching the first hello-world commit vs fetching the latest, and the resulting "cache directory" size/increase:

$ BASHBREW_CACHE="$HOME/temp/tmp/bashbrew-cache" ~/docker/bashbrew/bashbrew.sh --debug from --uniq https://github.com/docker-library/official-images/raw/b04f64dd645433ff2c70882eb550660969bfb244/library/hello-world
warning: insecure protocol git:// detected: git://github.com/docker-library/hello-world
hello-world:latest: scratch

$ du -hs ~/temp/tmp/bashbrew-cache
64K	/home/tianon/temp/tmp/bashbrew-cache

$ BASHBREW_CACHE="$HOME/temp/tmp/bashbrew-cache" ~/docker/bashbrew/bashbrew.sh --debug from --uniq https://github.com/docker-library/official-images/raw/master/library/hello-world
hello-world:linux: scratch
hello-world:nanoserver-1809: mcr.microsoft.com/windows/nanoserver:1809

$ du -hs ~/temp/tmp/bashbrew-cache
632K	/home/tianon/temp/tmp/bashbrew-cache