Integrate capistrano/slackify with capistrano-magento2 deployments

Capfile

# Include gem used for Slack deploy notifications
require 'capistrano/slackify'

# Load custom tasks from `lib/capistrano/tasks` if you have any defined
Dir.glob('lib/capistrano/tasks/*.rake').each { |r| import r }

deploy.rb

set :slack_url, '<slack_webhook_url>'
set :slack_username, 'capistrano-magento2'
set :slack_channel, ['#channel-name']
set :slack_emoji, ':capistrano:'  # Custom emoji of capistrano logo uploaded to Slack
set :slack_deploy_pending_format, '--pretty="format:%h %ad %aN %s" --date=short'

# Change the label on 'stage' to avoid confusion
set :slack_custom_field_mapping, -> {
  {
    'stage' => {
      title: 'Environment',
      value: fetch(:stage),
      short: true
    }
  }
}

slackify.rake

include Capistrano::Magento2::Pending

module Slackify
  class PendingPayload < Payload
    def payload(channel)
      MultiJson.dump(
        {
          channel: channel,
          username: fetch(:slack_username),
          icon_emoji: fetch(:slack_emoji),
          text: fetch(:slack_pending_changes_text).join("\n"),
        }
      )
    end
  end
end

namespace :slack do
  task :notify_pending do
    set :slack_pending_changes_text, '>>>'
    on roles fetch(:magento_deploy_pending_role) do |host|
      ensure_revision true do
        # update local repository to ensure accuracy of report
        run_locally do
          execute :git, :fetch, :origin
        end

        # fetch current revision and revision to be deployed
        from = from_rev
        to = to_rev

        # if there is nothing to deploy on this host, inform the user
        if from == to
          append :slack_pending_changes_text, "*==> #{host}* (no changes to deploy)"
        else
          run_locally do
            append :slack_pending_changes_text, "*==> #{host}* (#{from} -> #{to})"

            # capture log of commits between current revision and revision for deploy
            output = capture :git, :log, "#{from}..#{to}", fetch(:slack_deploy_pending_format)

            # if we get no results, flip refs to look at reverse log in case of rollback deployments
            if output.to_s.strip.empty?
              output = capture :git, :log, "#{to}..#{from}", fetch(:slack_deploy_pending_format)
              if not output.to_s.strip.empty?
                append :slack_pending_changes_text,
                  "*Warning: It appears you may be going backwards in time with this deployment!*"
              end
            end

            # write pending changes log
            append :slack_pending_changes_text, "```#{output}```"
          end
        end
      end
    end
    # set :slack_pending_changes_text, pending_changes

    run_locally do
      info 'Notifying Slack of changelog pending deploy'
      Array(fetch(:slack_channel)).each {|channel|
        execute :curl, '-X POST', '-s', '--data-urlencode',
          Slackify::PendingPayload.build(self, :pending, channel),
          fetch(:slack_url)
      }
    end
  end
  after 'deploy:pending:log', 'slack:notify_pending'
end