Optionally include VCS metadata when doing `cabal sdist`

Suppose you are using the package `foo-X`, and you think you have found a bug in it. At this point you typically want to go to the source of `foo-X` and have a look. There is `cabal get` which will get you the source, but often you'd rather have the actual _repository_ at the corresponding commit. But there's no easy way to go from a cabal package to the repository+commit that it was built from. In practice the process is something like: go to Hackage, find the link to the repo, go to the repo, hope they tag their releases... which some people don't.

That seems perfectly reasonable: a cabal package is just built from a source distribution, which doesn't include that information. But we could potentially include the revision as optional metadata when we _build_ the source distribution. That would allow downstream tools to get it out if they needed to. For example, for repositories hosted on Github, Hackage could provide a link to the exact commit.

I'm not sure what the right way to do this would be, perhaps by writing an `x-git-rev` field into the cabal file when sdisting?