Drop all tables before restoring a PostgreSQL DB

Invoking 'db:schema:load' turned out to be a bad idea: when downgrading
an existing GitLab installation, the schema of the newer version would
be preserved when trying to import the old version.

CHANGELOG

@@ -2,6 +2,7 @@ v 6.8.0
   - Ability to at mention users that are participating in issue and merge req. discussion
   - Enabled GZip Compression for assets in example Nginx, make sure that Nginx is compiled with --with-http_gzip_static_module flag (this is default in Ubuntu)
   - Make user search case-insensitive (Christopher Arnold)
+  - Drop all tables before restoring a Postgres backup
 v 6.7.2
   - Fix upgrader script

lib/backup/database.rb

@@ -29,9 +29,10 @@ module Backup
         print "Restoring MySQL database #{config['database']} ... "
         system('mysql', *mysql_args, config['database'], in: db_file_name)
       when "postgresql" then
-        puts "Destructively rebuilding database schema for RAILS_ENV #{Rails.env}"
-        Rake::Task["db:schema:load"].invoke
         print "Restoring PostgreSQL database #{config['database']} ... "
+        # Drop all tables because PostgreSQL DB dumps do not contain DROP TABLE
+        # statements like MySQL.
+        Rake::Task["gitlab:db:drop_all_tables"].invoke
         pg_env
         system('psql', config['database'], '-f', db_file_name)
       end