feat: Ensure non-shallow submodules

[Muscraft]

rust-lang/rust switched to shallow clones for submodules and it started causing problems with subup. The two I have encountered are:

1. When updating the cargo submodule, the PR description it would create would not include all of the correct commit, but they would be in the actual commit
   • This is likely due to the shallow clone not including all of the required info
2. When doing a beta backport of the cargo submodule, it would fail as it couldn't find the revision

   Status: Checking submodule revs.
   fatal: Needed a single revision
   fatal: Needed a single revision
   Error: Failed to determine rev `rust-1.79.0` for path `src/tools/cargo`
   Caused by: Failed to run command: git rev-parse --verify rust-1.79.0
   Caused by: Command exit status Some(128)
   

After much testing, it appears both of these are caused by shallow clones and can be fixed by unshallowing the submodules. One way to do this is to use git fetch --unshallow, which is the approach I took in this PR. One thing to note with --unshallow is it can only be run on shallow clones.

[ehuss]

Thanks!

cc @weihanglo FYI