$ git clone -n https://src.fedoraproject.org/rpms/perl-Math-BigInt-FastCalc.git /var/lib/mock/f40-build-2212778-52289/root/chroot_tmpdir/scmroot/perl-Math-BigInt-FastCalc
Cloning into '/var/lib/mock/f40-build-2212778-52289/root/chroot_tmpdir/scmroot/perl-Math-BigInt-FastCalc'...
$ git fetch origin de5f89ce201cdd395778e3df9e7df0e6d2233135:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/perl-Math-BigInt-FastCalc
 * [new ref]         de5f89ce201cdd395778e3df9e7df0e6d2233135 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at de5f89c 0.5018 bump (rhbz#2257058)