pkg/vcs: implement remaining Repo methods for fuchsia

This is a temporary workaround for managing a
fuchsia checkout:
- Mark the repo as "precious" so that checkout
  operations don't delete GN args and other build
  configuration state. In a follow-up, this will
  be replaced by a fuchsia-specific `repair` method
  using `fx clean`.
- Checkout operations apply only to fuchsia.git, which
  has a chance of creating inconsistencies between
  versions of fuchsia and its dependencies. The main
  poll workflow (using `jiri`) is not affected by this
  problem, but bisection workflows may fail. This will
  be fixed in a follow-up by checking out branches and
  commits from fuchsia's global integration repo.

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