$ git clone -n https://src.fedoraproject.org/rpms/R-jsonlite.git /var/lib/mock/f42-build-side-1-3500312-45970/root/chroot_tmpdir/scmroot/R-jsonlite
Cloning into '/var/lib/mock/f42-build-side-1-3500312-45970/root/chroot_tmpdir/scmroot/R-jsonlite'...
$ git fetch origin e571b74c8d7ba38b4bb533aa80c7253168c65946:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-jsonlite
 * [new ref]         e571b74c8d7ba38b4bb533aa80c7253168c65946 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at e571b74 R-maint-sig mass rebuild