API extension for Git hashes

The Justbuild authors recently published a modified version of the API, with improvements for Git-specific use cases. @sluongng [suggested](https://groups.google.com/g/bazel-discuss/c/SM06kOTvNwQ/m/Bo5IGQErAwAJ) to open an issue to discuss whether the API could be further improved in this regard.

[Justbuild](https://github.com/just-buildsystem/justbuild) already implements this extended API, both server and client side. It's specification can be found here: https://github.com/just-buildsystem/justbuild/blob/master/doc/specification/remote-protocol.org
This document also discusses the improvements introduced by this change.

Oliver

Start by reading the Justbuild remote protocol specification linked in the issue and the issue's discussion of its Git-specific API improvements. Compare those changes with the existing remote API and document a concrete proposal for any extension, including its scope and compatibility requirements.

Written by the indexing model from the issue text.