Carry over `CABAL_CHDEPS` from previous versions

[hololeap]

It would be nice if hackport could automatically carry over CABAL_CHDEPS when running hackport merge and modify RDEPEND/DEPEND accordingly.

+ CABAL_CHDEPS=(
+ 	'hedgehog                   >= 1.0        && < 1.4' 'hedgehog >= 1.0'
+ )

- >=dev-haskell/hedgehog-1.0 <dev-haskell/hedgehog-1.4
+ >=dev-haskell/hedgehog-1.0

Ideally, it would be able to detect if the snippet changed by CABAL_CHDEPS was still in the .cabal file and decide which parts to carry over based on that.