$ git clone -n https://src.fedoraproject.org/rpms/libcxx.git /var/lib/mock/f31-build-99265-33139/root/chroot_tmpdir/scmroot/libcxx
Cloning into '/var/lib/mock/f31-build-99265-33139/root/chroot_tmpdir/scmroot/libcxx'...
$ git reset --hard e83b27391abe534323f92978b984aec69f3ad1b4
HEAD is now at e83b273 - Rebuilt for https://fedoraproject.org/wiki/Fedora_31_Mass_Rebuild