Optional VCS Support in `cabal sdist` (and all-in-one updist workflow)

It would be nice to have a one-step process for releasing packages. Suppose we had `cabal sdist --vcs`, initially configured to work with git alone, but easily extensible.

Running this would rely on the upstream repo in the cabal file being set correctly. Having done so it would:

* Confirm that the source-repo state matched the head of the branch
* Tag the branch, update the cabal file to point to the correct tag (using the sadly underused `tag` field component of the repo field), and push.
* build the tarball as usual with sdist

We could also add a `--test` flag that would unpack the tarball and attempt to build it (and if it has a test stanza, test it).

We could then add a new command, say `updist` that by default in order would `cabal sdist --test; cabal sdist --vcs; cabal sdist; cabal upload`, perhaps with flags to omit workflow steps. Not by default, but optionally it could have a flag to build and upload haddocks as well.

This is just quality of life, but I think it would make things much nicer, as the steps are all so mechanical, and always the same.