Support checkout of specific file/path.

[bwplotka]

Hi :wave:

I am pretty sure with existing API we are missing git checkout -- <file> https://github.com/src-d/go-git/issues/891 feature. I would like to propose:

• Extending CheckoutOptions with Path string which will make checkout behavior as here: https://git-scm.com/docs/git-checkout#Documentation/git-checkout.txt-emgitcheckoutem-f--ours--theirs-m--conflictltstylegtlttree-ishgt--pathspec-from-fileltfilegt--pathspec-file-nul

Would you accept such contribution? :hugs:

[missedone]

I'm looking for this feature "Sparse Checkout" as well. anybody already worked on this? thanks