$ git clone -n https://src.fedoraproject.org/rpms/R-pkgcache.git /var/lib/mock/f38-build-724200-99404/root/chroot_tmpdir/scmroot/R-pkgcache
Cloning into '/var/lib/mock/f38-build-724200-99404/root/chroot_tmpdir/scmroot/R-pkgcache'...
$ git fetch origin a81119bb15e16a09a5690554beee6edf101b3f9d:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-pkgcache
 * [new ref]         a81119bb15e16a09a5690554beee6edf101b3f9d -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at a81119b R-maint-sig mass rebuild