libfetchers/git: restore compat with builtins.fetchGit from 2.3

Since fb38459d6e, each `ref` is appended
with `refs/heads` unless it starts with `refs/` already. This regressed
two use-cases that worked fine before:

* Specifying a commit hash as `ref`: now, if `ref` looks like a commit
  hash it will be directly passed to `git fetch`.

* Specifying a tag without `refs/tags` as prefix: now, the fetcher prepends
  `refs/*` to a ref that doesn't start with `refs/` and doesn't look
  like a commit hash. That way, both a branch and a tag specified in
  `ref` can be fetched.

  The order of preference in git is

  * file in `refs/` (e.g. `HEAD`)
  * file in `refs/tags/`
  * file in `refs/heads` (i.e. a branch)

  After fetching `refs/*`, ref is resolved the same way as git does.

Change-Id: Idd49b97cbdc8c6fdc8faa5a48bef3dec25e4ccc3

src/libexpr/primops/fetchTree.cc

@@ -394,7 +394,8 @@ static RegisterPrimOp primop_fetchGit({
         [Git reference]: https://git-scm.com/book/en/v2/Git-Internals-Git-References
 
         By default, the `ref` value is prefixed with `refs/heads/`.
-        As of 2.3.0, Nix will not prefix `refs/heads/` if `ref` starts with `refs/`.
+        As of 2.3.0, Nix will not prefix `refs/heads/` if `ref` starts with `refs/` or
+        if `ref` looks like a commit hash for backwards compatibility with CppNix 2.3.
 
       - `submodules` (default: `false`)