Update columns in batches until no rows are left

Instead of updating a fixed number of rows (based on the amount of rows
available at the start of the update) the method
"update_column_in_batches" will now continue updating rows until it runs
out of rows to process.

For a table with a high rate of inserts this may result in the migration
taking quite some time. However, the alternative is not all rows being
updated or the "change_column_null" method raising an error due to there
being NULL values.

lib/gitlab/database/migration_helpers.rb

@@ -55,10 +55,10 @@ module Gitlab
           first['count'].
           to_i
 
-        # Update in batches of 5%
+        # Update in batches of 5% until we run out of any rows to update.
         batch_size = ((total / 100.0) * 5.0).ceil
 
-        while processed < total
+        loop do
           start_row = exec_query(%Q{
             SELECT id
             FROM #{quoted_table}
@@ -66,6 +66,9 @@ module Gitlab
             LIMIT 1 OFFSET #{processed}
           }).to_hash.first
 
+          # There are no more rows to process
+          break unless start_row
+
           stop_row = exec_query(%Q{
             SELECT id
             FROM #{quoted_table}