* Fetch full history of tags (in particular, `current-hackage`).
* Before fetching tags, transition previously shallow repos to be
non-shallow with `fetch --unshallow`.
* Fetch full history in initial clone, otherwise, we immediately
afterwards use `fetch --unshallow`.
This means that the initial fetch and later updates require more data,
proportional to the entire repository history; however, reducing data
usage again is not trivial and would require changes in the layout of
all-cabal-hashes, as discussed in #2175.