}
sub _isHash {
    my ($rev) = @_;
    return length($rev) == 40 && $rev =~ /^[0-9a-f]+$/;

        # TODO: Optimize the first clone by using "git init $clonePath" and "git remote add origin $uri".
        $res = run(cmd => ["git", "clone", "--branch", $branch, $uri, $clonePath], timeout => 600);
        die "error cloning git repo at `$uri':\n$res->{stderr}" if $res->{status};

        $res = run(cmd => ["git", "init", $clonePath]);
        $res = run(cmd => ["git", "remote", "add", "origin", "--", $uri], dir => $clonePath) unless $res->{status};
        die "error creating git repo in `$clonePath':\n$res->{stderr}" if $res->{status};

    $res = run(cmd => ["git", "fetch", "-fu", "origin", "+$branch:$branch"], dir => $clonePath, timeout => 600);

    my $localBranch = _isHash($branch) ? "_hydra_tmp" : $branch;
    $res = run(cmd => ["git", "fetch", "-fu", "origin", "+$branch:$localBranch"], dir => $clonePath, timeout => 600);

    my $revision = grab(cmd => ["git", "rev-parse", "$branch"], dir => $clonePath, chomp => 1);

    my $revision = _isHash($branch) ? $branch
        : grab(cmd => ["git", "rev-parse", "$branch"], dir => $clonePath, chomp => 1);

            # script.  Thus, we leave `.git' in there.  Same for
            # Subversion (e.g., libgcrypt's build system uses that.)

            # script.  Thus, we leave `.git' in there.

        # FIXME: Don't use nix-prefetch-git.