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