Fix `alias.ff = merge --ff-only` #ImAnIdiot

`alias.ff` was added in 901ece2e03181ad02d73ae4796418ed1c25d7fd5 along
with `merge.ff = only`, which actually meant `alias.mg = merge --no-ff`
wouldn't work at all. I tried to fix this by doing `pull.ff = only`
instead in eb869d7e673399d3a7406e9675ce2cf3d22a54db (at which point I
should also have done this, i.e. made `alias.ff = merge --ff-only`),
although that never worked, there's no `pull.ff` option, and I
eventually had to add `alias.pl = pull --ff-only`,
8624dbf3b410d2f675727c009f1da3e6463032d0.