$ git clone -n https://src.fedoraproject.org/rpms/rust-base64_0.21.git /var/lib/mock/f45-build-40396-771/root/chroot_tmpdir/scmroot/rust-base64_0.21
Cloning into '/var/lib/mock/f45-build-40396-771/root/chroot_tmpdir/scmroot/rust-base64_0.21'...
$ git fetch origin 15e4c89d2b1e737dc2f14396f138951d443ece17:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/rust-base64_0.21
 * [new ref]         15e4c89d2b1e737dc2f14396f138951d443ece17 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 15e4c89 Rebuilt for https://fedoraproject.org/wiki/Fedora_45_Mass_Rebuild