Change merge_error column from string to text type

In some cases, the artificial 255-character limit would cause an error in
MergeWorker.

Closes #20593

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
+
 v 8.12.0 (unreleased)
+  - Change merge_error column from string to text type
   - Optimistic locking for Issues and Merge Requests (title and description overriding prevention)
 
 v 8.11.0 (unreleased)

db/migrate/20160823081327_change_merge_error_to_text.rb

+class ChangeMergeErrorToText < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = true
+  DOWNTIME_REASON = 'This migration requires downtime because it alters a column from varchar(255) to text.'
+
+  def change
+    change_column :merge_requests, :merge_error, :text, limit: 65535
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160819221833) do
+ActiveRecord::Schema.define(version: 20160823081327) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -613,7 +613,7 @@ ActiveRecord::Schema.define(version: 20160819221833) do
     t.integer  "position",                     default: 0
     t.datetime "locked_at"
     t.integer  "updated_by_id"
-    t.string   "merge_error"
+    t.text     "merge_error"
     t.text     "merge_params"
     t.boolean  "merge_when_build_succeeds",    default: false, null: false
     t.integer  "merge_user_id"