$ git clone -n https://src.fedoraproject.org/rpms/firefox.git /var/lib/mock/f43-build-59016822-6570239/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f43-build-59016822-6570239/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin f027bb52f41706fbc2875cc3eaa2f48c2226aa90:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/firefox
 * [new ref]         f027bb52f41706fbc2875cc3eaa2f48c2226aa90 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at f027bb5 Added fix for rhbz#2358565