Original title: No evaluation of inode equality in FileContentsProxy

Description of the problem / feature request:

No evaluation of inode equality in FileContentsProxy.

Feature requests: what underlying problem are you trying to solve with this feature?

Speed up analysis phase by not adding whole worskspace to second level digest check. (described here).

What operating system are you running Bazel on?

Ubuntu 18.04/20.04.

What's the output of bazel info release?

release 2.2.0

Any other information, logs, or outputs that you want to share?

We are compiling a large repo (several MLoC) and the analysis phase takes 1-2 minutes.
We are using several layers of caching, including bazel remote cache. To reduce the traffic to the remote cache, we are also reusing .cache/bazel intermediate files across runs (baked into a node image deployed onto VMs).
Since the inodes of files after a git pull and checkout are not the same across runs, analysis phase takes very long, since files are effectively rehashed.