$ git clone -n https://src.fedoraproject.org/rpms/gthumb.git /var/lib/mock/f39-build-1562781-37462/root/chroot_tmpdir/scmroot/gthumb
Cloning into '/var/lib/mock/f39-build-1562781-37462/root/chroot_tmpdir/scmroot/gthumb'...
$ git fetch origin aeed1fdf0a07f87df26b823630d15f580a268bd8:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/gthumb
 * [new ref]         aeed1fdf0a07f87df26b823630d15f580a268bd8 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at aeed1fd Update to 3.12.4