Update to latest Cabal 1.18 branch tip (again)

This was already performed in 37d6e2c54f117 but accidentally
reverted as a side-effect of 9976c2ece3aa4.

So this update pulls in the fix into ghc-7.8 for #8266 again.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>