delete_all_repos.rb

#!/path/to/redmine/script/runner

Repository.all.each(&:"destroy!")

recreate_them.rb

#!/path/to/redmine/script/runner

Project.all.each do |project|
  # adapt the path!
  repo_path = "/path/to/repos/#{project.identifier}.git"

  unless File.exist?(repo_path)
    puts "+++ Skipped #{project.name} (#{project.identifier})"
    next
  end

  project.repository = Repository.factory('Git', :url => repo_path)
  if project.repository && project.repository.save
    puts "+++ Created #{project.name} (#{project.identifier})"
  else
    puts "--- FAILED #{project.name} (#{project.identifier})"
  end

  # Now fetch all changesets
  project.repository.fetch_changesets
end