Add explicit support for git lfs

I would like to use git lfs and I would like to jenkins, and I would like for jenkins to use its same credentials already configured for git to fetch the git lfs resources. Rather than trying to change the init+fetch pattern, I would like to add a custom behavior to the git plugin that will execute `git lfs pull` after checkout using the same credentials as the fetch command.

It seems that git lfs is the way forward for git repos with large files and I don't see a workaround right now for using git lfs with jenkins, short of sprinkling credentials everywhere so that `git lfs` can be run from the build context which doesn't seem like a good idea.

I am looking into what it would take to add this. Would the maintainers be interested in taking in such a feature?