Adding support to BuildStream for git lfs repositories

[Richard Dale <richard dale codethink co uk>]

Please find attached a patch for adding git lfs support to BuildStream. 
As I say in the commit message, I think it is the wrong approach:

"Use 'git lfs ls-files <ref>' to determine whether
or not there are any files under lfs control in
the target branch.

If there are files stored in git lfs, then change
the origin URL to be the upstream one, rather than
the local cached bare repo.

This approach works, but it would be better to leave
the origin URL, and instead add an .lfsconfig file
that points to the lfs object store. However, I couldn't
find a way of doing this without get a user/password
prompt from git."

But it does work in the project I am building with BuildStream, and I 
hope it can be a starting point for a discussion.

--
Richard Dale
Software Developer - Codethink
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Attachment: 0001-Add-support-for-Git-LFS-repositories.patch
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