bug: unable to determine the branch associated with the rev git depends on

[jetjinser]

If rev is not in the HEAD positioned branch, builtins.fetchGit cannot find it.

To reproduce

minimal case: https://github.com/jetjinser/madsim-br

the point is:

[dependencies]
tokio-stream = { git = "https://github.com/madsim-rs/tokio.git", rev = "fe39bb8e" }

the madsim-rs/tokio@fe39bb8e in branch madsim.

nix build

gets error like:

error: Cannot find Git revision 'fe39bb8e8ab0ed96ee1b4477ab5508c20ce017fb' in ref 'HEAD' of repository 'https://github.com/madsim-rs/tokio.git'! Please make sure that the rev exists on the ref you've specified or add allRefs = true; to fetchGit.