This is a reland of 19512276.
It includes an update to .gitattributes for recipes.

Original change's description:
> repo: auto-update depot_tools when used
>
> Since gclient automatically syncs this checkout when used,
> have repo do the same for CrOS users.
>
> Bug: 1148984
> Test: ran and saw it auto-update when used externally
> Test: ran and saw it not auto-update when used inside of a repo checkout
> Change-Id: I56b9a45ef3a9ed501f6277e8f8357d4719cf2557
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2537164
> Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Michael Mortensen <mmortensen@google.com>
> Commit-Queue: Mike Frysinger <vapier@chromium.org>

Bug: 1148984
Change-Id: Ia7844016cb04d3197e51ad05da64c885cf3277a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2561229Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>