$ git clone -n https://src.fedoraproject.org/rpms/rust-git2_0.17.git /var/lib/mock/f40-build-2651020-66878/root/chroot_tmpdir/scmroot/rust-git2_0.17
Cloning into '/var/lib/mock/f40-build-2651020-66878/root/chroot_tmpdir/scmroot/rust-git2_0.17'...
$ git fetch origin bec05988b0fd0d354c164866c7606c5ab7f9f6c6:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/rust-git2_0.17
 * [new ref]         bec05988b0fd0d354c164866c7606c5ab7f9f6c6 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at bec0598 Initial import (git2 0.17 compat package)