RFC: Add `--git-repos` flag to `cargo update`

I've observed a case where a developer was pinning everything inside of Cargo.toml except their Git Repositories for the mere benefit of having an easy method to update all of their git dependencies. All of their internal crates were in git repositories. That is to say all of their versions strings where like =0.1.2. This presents a problem as you have to sacrifice the intended functionality of cargo update which can provide security patches and ease upgrades.

Currently, there is a better way to do this with cargo update -p foo -p bar. It would be nice if we could do cargo update --git-repos which would update only the git repositories such that effectively everything else was ignored (or pinned to the current version).

The issue names the cargo update command and the proposed --git-repos flag, but no files, tests, or implementation entry points. Start by researching how cargo update -p selects dependencies and how the RFC process captures command behavior. Done means an accepted design that specifies which dependencies are updated and which remain pinned.