Merge branch 'overwrite-during-backup' into 'master'

Don't stop if database.sql.gz already exists

The existing behavior of the backups is to overwrite whatever data
was still there in the scratch directories. This broke when we added
a 'gzip' step because 'gzip database.sql' will fail if 'database.sql.gz'
already exists. Doing 'rm -f database.sql.gz' before the 'gzip'
avoids this failure.

See merge request !1011

lib/backup/database.rb

@@ -25,6 +25,7 @@ module Backup
       abort 'Backup failed' unless success
 
       $progress.print 'Compressing database ... '
+      FileUtils.rm_f db_file_name_gz
       success = system('gzip', db_file_name)
       report_success(success)
       abort 'Backup failed: compress error' unless success