Consider deduplicating digests for hardlinked/symlinked outputs

[tjgq]

Currently, when an action copies inputs to outputs by hardlinking or symlinking them, we recalculate digests for the output files, which can take a significant performance toll on builds that copy lots of files around. This could in theory be avoided if Bazel was able to recognize that it has seen the target of the link before (and hasn't been modified since).

For hardlinks, this could be as simple as using (st_dev, st_ino, st_mtime, st_ctime, st_size) as the DigestUtils cache key (instead of the current (path, st_ino, st_mtime, st_ctime, st_size)). Symlinks would require a bit more work, but are still doable. We'd have to think very carefully about the correctness implications, though.

cc @woody77 @fangism