$ git clone -n https://src.fedoraproject.org/rpms/mercurial.git /var/lib/mock/f40-build-2153632-50914/root/chroot_tmpdir/scmroot/mercurial
Cloning into '/var/lib/mock/f40-build-2153632-50914/root/chroot_tmpdir/scmroot/mercurial'...
$ git fetch origin 91781940a9d984158e5f32f9f6e1fc011986a14c:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/mercurial
 * [new ref]         91781940a9d984158e5f32f9f6e1fc011986a14c -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 9178194 mercurial 6.6.1