[7.1.0] Avoid a superfluous stat() in DigestUtil.

[tjgq]

Similar in spirit to https://github.com/bazelbuild/bazel/commit/bb8031989397633326520720013f23eff91d8a42. DigestUtil wraps DigestUtils, trading in Digests instead of byte arrays.

There are probably other callsites that would benefit from the optimization, but I'm focused on UploadManifest at the moment.

PiperOrigin-RevId: 607695005 Change-Id: Iaa9b1f07b26f05482cb6f3fe8dfaacf50e0c07f5