$ git clone -n https://src.fedoraproject.org/rpms/R-gmp.git /var/lib/mock/f42-build-side-1-3501721-45976/root/chroot_tmpdir/scmroot/R-gmp
Cloning into '/var/lib/mock/f42-build-side-1-3501721-45976/root/chroot_tmpdir/scmroot/R-gmp'...
$ git fetch origin 2db7e3a58930e82c1a6fa844962d2933acbbfca7:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-gmp
 * [new ref]         2db7e3a58930e82c1a6fa844962d2933acbbfca7 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 2db7e3a R-maint-sig mass rebuild