$ git clone -n https://src.fedoraproject.org/rpms/notmuch.git /var/lib/mock/f39-build-1640606-40147/root/chroot_tmpdir/scmroot/notmuch
Cloning into '/var/lib/mock/f39-build-1640606-40147/root/chroot_tmpdir/scmroot/notmuch'...
$ git fetch origin 4938382d89998e61ff332ca9d5005d9677615e7e:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/notmuch
 * [new ref]         4938382d89998e61ff332ca9d5005d9677615e7e -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 4938382 rebase with upstream release 0.38 (rhbz#2234464)