gitcheckout: add ability to checkout files from branch without switching

[franzheidl]

It would be great to have the ability to check out files from another branch without switching branches like you can do in git:

git checkout branchname -- filename

Ideally it would be possible to add a files option to gitcheckout to do that:

gitcheckout: {
    your_target: {
        options: {
            branch: 'branchname',
            files: ['filename', 'otherfilename']
        }
    }
}

[pajtai]

and git checkout also works w sha refs, so that'd be nice too