Git fetcher: Don't use refspec <rev>:<rev>

This causes Git to create a local ref named refs/head/<rev>, e.g.

  $ git -C ~/.cache/nix/gitv3/11irpim06vj4h6c0w8yls6kx4hvl0qd0gr1fvk47n76g6wf1s1vk ls-remote --symref .
  5c4410e3b9891c05ab40d723de78c6f0be45ad30        refs/heads/5c4410e3b9891c05ab40d723de78c6f0be45ad30
  7f6bde8a20de4cccc2256f088bc5af9dbe38881d        refs/heads/7f6bde8a20de4cccc2256f088bc5af9dbe38881d

which confuses readHead(), leading to errors like

  fatal: Refusing to point HEAD outside of refs/
  warning: could not update cached head 'd275d93aa0bb8a004939b2f1e87f559f989453be' for 'file:///tmp/repo'

Author: edolstra

src/libfetchers/git.cc

@@ -611,16 +611,16 @@ struct GitInputScheme : InputScheme
                 try {
                     auto fetchRef =
                         getAllRefsAttr(input)
-                        ? "refs/*"
+                        ? "refs/*:refs/*"
                         : input.getRev()
                         ? input.getRev()->gitRev()
                         : ref.compare(0, 5, "refs/") == 0
-                        ? ref
+                        ? fmt("%1%:%1%", ref)
                         : ref == "HEAD"
                         ? ref
-                        : "refs/heads/" + ref;
+                        : fmt("%1%:%1%", "refs/heads/" + ref);
 
-                    repo->fetch(repoUrl.to_string(), fmt("%s:%s", fetchRef, fetchRef), getShallowAttr(input));
+                    repo->fetch(repoUrl.to_string(), fetchRef, getShallowAttr(input));
                 } catch (Error & e) {
                     if (!std::filesystem::exists(localRefFile)) throw;
                     logError(e.info());