Set file modification time on checkout

[Patrick Hemmer <git@stormcloud9.net>]

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