RFE: Add support to git LFS to the git source plugin

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/567)
In GitLab by [[Gitlab user @jjardon]](https://gitlab.com/jjardon) on Aug 9, 2018, 10:43

git lfs is a Git extension for versioning large files: https://git-lfs.github.com/

If I try to use a git lfs repo at the moment, this is the error I'm getting:
```
Downloading bzip2-1.0.6.tar.bz2 (709 KB)
Error downloading object: bzip2-1.0.6.tar.bz2 (d70a9cc): Smudge error: Error downloading bzip2-1.0.6.tar.bz2 (d70a9ccd8bdf47e302d96c69fecd54925f45d9c7b966bb4ef5f56b770960afa7): batch request: missing protocol: "/home/jjardon/.cache/buildstream/sources/git/https___gitlab_com_freedesktop_sdk_bzip2_tarball_git.git/info/lfs"

Errors logged to /home/jjardon/.cache/buildstream/build/bzip2-z6uk2y5j/root/buildstream/freedesktop-sdk-bootstrap/bzip2.bst/.git/lfs/logs/20180809T103920.716221677.log
Use `git lfs logs last` to view the log.
error: external filter 'git-lfs smudge -- %f' failed 2
error: external filter 'git-lfs smudge -- %f' failed
fatal: bzip2-1.0.6.tar.bz2: smudge filter lfs failed
[00:00:00] FAILURE bzip2.bst-0: Staging https://gitlab.com/freedesktop-sdk/bzip2-tarball.git
[00:00:00] FAILURE bzip2.bst: Staging sources
[00:00:05] FAILURE bzip2.bst: git source at bzip2.bst [line 55 column 4]: Failed to checkout git ref 8cdcb115dd45dd5d0f795f0d6af55a1054604fdb
```

git source plugin から始め、issue に記載された GitLab リポジトリを使って checkout の失敗を再現します。Git LFS リポジトリをどのように取得して checkout すべきかを特定し、bzip2 の例が smudge-filter エラーなしで完了することを確認します。

索引モデルが issue の本文から書いたものです。