Merge branch 'xl/upgrade-repo-format'

Allow runtime upgrade of the repository format version, which needs
to be done carefully.

There is a rather unpleasant backward compatibility worry with the
last step of this series, but it is the right thing to do in the
longer term.

* xl/upgrade-repo-format:
  check_repository_format_gently(): refuse extensions for old repositories
  sparse-checkout: upgrade repository to version 1 when enabling extension
  fetch: allow adding a filter after initial clone
  repository: add a helper function to perform repository format upgrade

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