$ git clone -n https://github.com/xcp-ng-rpms/gmp /var/lib/mock/v8.3-incoming-10909-29394/root/chroot_tmpdir/scmroot/gmp
Cloning into '/var/lib/mock/v8.3-incoming-10909-29394/root/chroot_tmpdir/scmroot/gmp'...
$ git fetch origin c741018a1a50600e87df74c6a855be98fae6e515:KOJI_FETCH_HEAD
From https://github.com/xcp-ng-rpms/gmp
 * [new ref]         c741018a1a50600e87df74c6a855be98fae6e515 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at c741018 Import gmp-6.2.1-5.xs8.src.rpm