$ git clone -n https://src.fedoraproject.org/rpms/thunderbird.git /var/lib/mock/f41-build-820174-152085/root/chroot_tmpdir/scmroot/thunderbird
Cloning into '/var/lib/mock/f41-build-820174-152085/root/chroot_tmpdir/scmroot/thunderbird'...
$ git fetch origin ca1ac17cbe02443e4a2c83eb008f52b8ff7d6918:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/thunderbird
 * [new ref]         ca1ac17cbe02443e4a2c83eb008f52b8ff7d6918 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at ca1ac17 Remove plugin-container as no longer shipped