Allow specifying a non-branch/non-tag ref for git dependencies

[Nemo157]

Describe the problem you are trying to solve
Adding a patch section referring to a reference that is not a branch or tag of the repository. For example refs/pull/<id>/head to refer to a pull request on Github.

Describe the solution you'd like
An additional ref key to go with branch, tag and rev that allows passing in a fully-qualified reference to lookup.

Notes
This may be slightly complicated as it appears that Cargo does a fetch of just refs/heads/*:refs/heads/* before looking up the specified branch locally. It might be better in any case to only fetch the requested ref from the remote to reduce traffic on repositories that have a large number of branches.

[Nemo157]

What I mention in the note would also allow accessing revisions that are not ancestors of any public branch/tag as long as they are accessible in the remote repository. Basically performing a git fetch <repository> <rev> or git fetch <repository> <ref>:<ref> to only pull the single commit and ancestors before looking it up locally.

[ehuss]

This should now be implemented (via #9859 now in 1.57) to fetch any ref with the syntax rev="refs/...":

[dependencies]
cargo = { git = "https://github.com/rust-lang/cargo.git", rev = "refs/pull/9859/head" }

I'm going to close as resolved, though feel free to reopen if I have misinterpreted the request.