Fix rebase commit SHA migration name

This already existed in EE with a different timestamp, so rename for CE and make
it idempotent.

db/migrate/20171230123729_add_rebase_commit_sha_to_merge_requests.rb

-class AddRebaseCommitShaToMergeRequests < ActiveRecord::Migration
-  DOWNTIME = false
-
-  def change
-    add_column :merge_requests, :rebase_commit_sha, :string
-  end
-end

db/migrate/20171230123729_add_rebase_commit_sha_to_merge_requests_ce.rb

+class AddRebaseCommitShaToMergeRequestsCe < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    unless column_exists?(:merge_requests, :rebase_commit_sha)
+      add_column :merge_requests, :rebase_commit_sha, :string
+    end
+  end
+
+  def down
+    if column_exists?(:merge_requests, :rebase_commit_sha)
+      remove_column :merge_requests, :rebase_commit_sha
+    end
+  end
+end