Support both ref & rev in flake input url

[dpc]

Describe the bug

Currently when I try to use both ref and ref in flake input url I get:

error: URL 'github:ipetkov/crane?ref=v0.12.1&rev=445a3d222947632b5593112bb817850e8a9cf737' contains both a commit hash and a branch/tag name v0.12.1 445a3d222947632b5593112bb817850e8a9cf737

Steps To Reproduce

Use an flake input url that has both ref and rev (that point at the same commit).

Expected behavior

I think it should be supported. nix should just verify that that ref is still pointing at that rev.

rev is for pinning to a hash for security/reliability, while ref is for letting humans reading the code know which release version is it. One could put the ref in a comment, but then there's nothing to make sure it's not incorrect.

[a-h]

nix flake clone is pretty useless without changing this.

If I try to clone a specific commit of a flake without a "ref" of main, I get a failure.

nix flake clone "github:numtide/system-manager/e8957ab8b4cf02574adb5f09ef4a2ef9ee48ef01" --dest local 
error: Git input 'git+https://github.com/numtide/system-manager.git?rev=e8957ab8b4cf02574adb5f09ef4a2ef9ee48ef01' has a commit hash but no branch/tag name

If I add in the branch of main, and set the rev, it complains.

nix flake clone "github:numtide/system-manager/main?rev=e8957ab8b4cf02574adb5f09ef4a2ef9ee48ef01" --dest local
error: URL 'github:numtide/system-manager/main?rev=e8957ab8b4cf02574adb5f09ef4a2ef9ee48ef01' contains both a commit hash and a branch/tag name main e8957ab8b4cf02574adb5f09ef4a2ef9ee48ef01

Here's where the error is thrown.

https://github.com/NixOS/nix/blame/5633c0975b9dec6bceaa85003223a346d7d1bd0b/src/libfetchers/git.cc#L114

[johannwagner]

We also ran into an issue by using builtins.fetchTree, which requires to set rev and ref to have a locked input. This also triggers this mentioned assertion. If we remove rev, the input is not locked anymore and cannot be used without --impure. If we omit ref, the commit hash is not found within our repository.

Example:

builtins.fetchTree {
  type = "gitlab";
  owner = "wobcom/nix";
  repo = "very-nice-repository";
  host = "git.wobcom.de";
  ref = "very-nice-branch";
  rev = "abcefac8153fce1ae8bb103ba2258f08614392d3";
};