$ git clone -n https://src.fedoraproject.org/rpms/glibmm2.4.git /var/lib/mock/f42-build-side-1-3500750-45971/root/chroot_tmpdir/scmroot/glibmm2.4
Cloning into '/var/lib/mock/f42-build-side-1-3500750-45971/root/chroot_tmpdir/scmroot/glibmm2.4'...
$ git fetch origin 56f49a9e80b4c68a5ed4e7f4971b12b882a52579:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/glibmm2.4
 * [new ref]         56f49a9e80b4c68a5ed4e7f4971b12b882a52579 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 56f49a9 Update to 2.66.8