$ git clone -n https://src.fedoraproject.org/rpms/firefox.git /var/lib/mock/f38-build-side-42-init-devel-657336-26596/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f38-build-side-42-init-devel-657336-26596/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin d13c7dc2b6f98a138c6dd67d26e891e0b452c319:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/firefox
 * [new ref]         d13c7dc2b6f98a138c6dd67d26e891e0b452c319 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at d13c7dc Disabled libproxy support due to regressions (rhbz#2207469)