$ git clone -n https://src.fedoraproject.org/rpms/perl-Math-BigInt-FastCalc.git /var/lib/mock/f40-build-2018769-46457/root/chroot_tmpdir/scmroot/perl-Math-BigInt-FastCalc
Cloning into '/var/lib/mock/f40-build-2018769-46457/root/chroot_tmpdir/scmroot/perl-Math-BigInt-FastCalc'...
$ git fetch origin 00f87a67f6e0d27e55a6709c7a021994062441a5:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/perl-Math-BigInt-FastCalc
 * [new ref]         00f87a67f6e0d27e55a6709c7a021994062441a5 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 00f87a6 0.5015 bump (rhbz#2239928)