Set file modification time on checkout

The request is to allow git to set the file modification time on checkout to the commit-author-date of the commit which last modified the file. Yes I know this is in the FAQ, but the FAQ entry is missing an increasingly common use case: docker. When docker builds an image, it generates layers of images based on each build step. Adding a file would be a build step, and for this action it generates a hash based on the file modification time & content. Next time the image is built, if these haven't changed, the previously built layer is reused. And like git commit history, if a layer changes, the hash of all subsequent layers change and have to be rebuilt. So reusing layers can save a ton of time.

Now I'm not proposing that this be made the default action. The `make` use case is legitimate. But it would be nice to have an option for `checkout` and `reset` which toggles the behavior.

-Patrick