[Port semi-linear history from EE] Move migration to CE and `merge_method` method

app/models/project.rb

@@ -1567,13 +1567,25 @@ class Project < ActiveRecord::Base
   def merge_method
     if self.merge_requests_ff_only_enabled
       :ff
+    elsif self.merge_requests_rebase_enabled
+      :rebase_merge
     else
       :merge
     end
   end
   def merge_method=(method)
-    self.merge_requests_ff_only_enabled = method.to_s == "ff"
+    case method.to_s
+    when "ff"
+      self.merge_requests_ff_only_enabled = true
+      self.merge_requests_rebase_enabled = true
+    when "rebase_merge"
+      self.merge_requests_ff_only_enabled = false
+      self.merge_requests_rebase_enabled = true
+    when "merge"
+      self.merge_requests_ff_only_enabled = false
+      self.merge_requests_rebase_enabled = false
+    end
   end
   def ff_merge_must_be_possible?

db/migrate/20141126120926_add_merge_request_rebase_enabled_to_projects.rb

+# rubocop:disable all
+class AddMergeRequestRebaseEnabledToProjects < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_column_with_default(:projects, :merge_requests_rebase_enabled, :boolean, default: false)
+  end
+
+  def down
+    remove_column(:projects, :merge_requests_rebase_enabled)
+  end
+end

db/schema.rb

@@ -1217,6 +1217,7 @@ ActiveRecord::Schema.define(version: 20170921115009) do
     t.boolean "resolve_outdated_diff_discussions"
     t.boolean "repository_read_only"
     t.boolean "merge_requests_ff_only_enabled", default: false
+    t.boolean "merge_requests_rebase_enabled", default: false, null: false
   end
   add_index "projects", ["ci_id"], name: "index_projects_on_ci_id", using: :btree