$ git clone -n https://github.com/xcp-ng-rpms/xen.git /var/lib/mock/v8.3-incoming-10806-29319/root/chroot_tmpdir/scmroot/xen
Cloning into '/var/lib/mock/v8.3-incoming-10806-29319/root/chroot_tmpdir/scmroot/xen'...
$ git fetch origin 15ba30aba29276f1fc794ba4f794b5cef5126c89:KOJI_FETCH_HEAD
From https://github.com/xcp-ng-rpms/xen
 * [new ref]         15ba30aba29276f1fc794ba4f794b5cef5126c89 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 15ba30a Merge pull request #82 from xcp-ng-rpms/dnt-XCPNG-2991