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
```

Commencez par le plugin source git et reproduisez l’échec du checkout avec le dépôt GitLab décrit dans l’issue. Déterminez comment les dépôts Git LFS doivent être récupérés et faire l’objet d’un checkout, puis vérifiez que l’exemple bzip2 s’achève sans l’erreur smudge-filter.

Rédigé par le modèle d'indexation à partir du texte de l'issue.