Add support for git repos with lfs

[uge]

Fixes #258

Modified src/sess.rs

Disable LFS smudging during the internal git clone / git checkout operation from the DB repo to the workspace checkout directory. This allows the checkout to succeed even with missing LFS objects. After checkout, configure lfs.url in the checkout directory to point to the original upstream repository URL.

Run git lfs pull to explicitly download the LFS objects from the upstream.

[micprog]

Thank you for this addition, I think it makes sense and is very useful in case lfs is enabled and needed!

Unfortunately, it will fail if git-lfs is not installed, which may be the case. Maybe we can disable the lfs functionality with the following:

• if there is a configuration in a bender configuration file (e.g., Bender.local) disabling this. This can be useful if the lfs files are known to not be required.
• if the repository doesn't require it. This way we save running the command and keep bender working for systems that don't currently have git-lfs installed.
• Maybe we can add a warning if it required but not supported (or disabled) instead of erroring out on the lfs comands completely.

My main concern is ensuring the existing systems and functionality do not break by adding this new feature. Happy to help out to make the required modifications.

I also see you added a file (bender-lfs-debug-top), I don't believe this is needed.

[uge]

Added functionality per points 2 and 3 to check for system install of git-lfs and also a warning if not supported but required. Also should only run if the repo has lfs in .gitattribute. Not sure how to structure a solution for the first point - should it be for the local repository or for the entire dependency tree?