explicitly define fix has_and_belongs_to_many join_table at Group and User

Rails4 cannot resolve automatically.

git-svn-id: http://svn.redmine.org/redmine/trunk@12471 e93f8b46-1217-0410-a6f0-8f06a7374b81
Rails4: scm: reload repository after destroying changesets in incremental fetch test

git-svn-id: http://svn.redmine.org/redmine/trunk@12472 e93f8b46-1217-0410-a6f0-8f06a7374b81
Rails4: replace deprecated find_by_id at Changeset class

git-svn-id: http://svn.redmine.org/redmine/trunk@12473 e93f8b46-1217-0410-a6f0-8f06a7374b81