Merge package in current directory by default

[hololeap]

If we are in (for instance) /var/db/repos/haskell/dev-haskell/cabal-install-parsers/, and hackport merge is run, it would be a convenient feature to have it implied that we mean hackport merge cabal-install-parsers.

This will probably need some sanity checks, but it could also just look at the top level of $PWD.

Thanks to @solpeth for bringing this up.