fix: better support dangling revisions

The submodules branch generates a checkout which depends on only the
references, not all revisions. If there are dangling revisions in the
bare fetcher clone (a correctness issue itself) then the subsequent
temporary directory clone will disastrously fail with messages like
```
fatal: reference is not a tree: 2da0785fa32ce4c628d501c8743c88be00835b50
```
You should be able to reproduce this behavior with
```bash

this_nixpkgs='github:nixos/nixpkgs?rev=36ac8d7e411eeb11ac0998d5a39e329c1226e541';
this_git=$(nix build --print-out-paths "$this_nixpkgs#git")/bin/git;

export GIT_AUTHOR_NAME=foo \
	[email protected] \
	GIT_AUTHOR_DATE=100000000 \
	GIT_COMMITER_NAME=foo \
	[email protected] \
	GIT_COMMITTER_DATE=100000000;

tmpgit=$(mktemp -d);
echo "Repo'll be at $tmpgit";

"$this_git" init "$tmpgit";

"$this_git" -C "$tmpgit" commit --allow-empty -m "foo"; # 8c2146823865b76da067b3bb458611a0a19ede3b
"$this_git" -C "$tmpgit" commit --allow-empty -m "foo"; # 2da0785fa32ce4c628d501c8743c88be00835b50
"$this_git" -C "$tmpgit" reset --hard HEAD~1; # 8c2146823865b76da067b3bb458611a0a19ede3b

for version in 18 19 20 21 22 23 24; do
    echo "Checking nix version 2.$version";
    this_nix=$(nix build "$this_nixpkgs#nixVersions.nix_2_$version^out" --print-out-paths)/bin/nix;

    url="file:///$tmpgit";

    # populate fetcher-v1.sqlite and the gitv3/ dir
    "$this_nix-build" --expr $'builtins.fetchGit {
        url = \"'"$url"$'\";
        rev = \"8c2146823865b76da067b3bb458611a0a19ede3b\";
        submodules = true;
    }';

    # put the "corrupted" git dir in place of the fetched one
    cache_dir="${XDG_CONFIG_HOME:-$HOME}"/.cache/nix/gitv3;
    tmp=$(mktemp);
    printf "%s" "$url" >"$tmp";
    fetched_dir="$cache_dir"/$(printf "%s" "$url" | $this_nix --extra-experimental-features nix-command hash file --type sha256 --base32 "$tmp");
    rm -rf "$fetched_dir";
    git clone --bare "$tmpgit" "$fetched_dir";

    # See if we can realise the derivation pointing to the dangling commit
    _NIX_FORCE_HTTP=1 "$this_nix-build" --impure --expr $'
    { pkgs ? (import <nixpkgs> { }) }: pkgs.stdenv.mkDerivation {
      pname = "foo";
      version = "1.0.0";
      dontUnpack = true;

      '"
      src = builtins.fetchGit {
        url = \"file:///$tmpgit\";
        rev = \"2da0785fa32ce4c628d501c8743c88be00835b50\";
        submodules = true;
      };

      buildPhase = ''
        touch \$out;
        exit 0;
      '';
    }
    "
done
```
This solution attempts to avoid the issue in the submodules path by
simply checking out the specified revision (in addition to all
references to preserve the original behavior - although, I think _only_
the revision is necessary).

src/libfetchers/git.cc

@@ -630,7 +630,7 @@ struct GitInputScheme : InputScheme
                 // everything to ensure we get the rev.
                 Activity act(*logger, lvlTalkative, actUnknown, fmt("making temporary clone of '%s'", repoDir));
                 runProgram("git", true, { "-C", tmpDir, "fetch", "--quiet", "--force",
-                        "--update-head-ok", "--", repoDir, "refs/*:refs/*" }, {}, true);
+                        "--update-head-ok", "--", repoDir, "refs/*:refs/*", input.getRev()->gitRev() }, {}, true);
             }
 
             runProgram("git", true, { "-C", tmpDir, "checkout", "--quiet", input.getRev()->gitRev() });