RFC consider including identifying infos from `ghc --info` in nix-style hash

An annoying issue when working with GHC HEAD or other custom GHCs is that they tend to have overlapping GHC version numbers if you build multiple versions of GHC on the same day.

One important identifying detail however if provided by `ghc --info`: the git commit id from which a GHC executable was derived, e.g.

```
"Project Git commit id","0156a3d815b784510a980621fdcb9c5b23826f1e"
```

In future, maybe Hadrian could provide additional identifying fingerprints to disambiguate different GHC builds from each other.

But I think introducing the Git commit id would already help avoiding a class of subtle ABI incompatibilties between GHC snapshots sharing the same version number.

Start by locating Cabal's nix-style hash generation and the handling of identifying information from `ghc --info`; the issue names no repository files or tests. Done means incorporating the GHC-derived Git commit id into the hash so custom or same-version GHC builds are distinguished, with tests covering the changed hash behavior.

Written by the indexing model from the issue text.