Move source_branch_exists property to non-cached entity
What does this MR do?
Moves source_branch_exists property to the mr widget entity which is not cached
https://gitlab.com/gitlab-org/gitlab-ce/issues/53817#note_210014513
Moves source_branch_exists property to the mr widget entity which is not cached
https://gitlab.com/gitlab-org/gitlab-ce/issues/53817#note_210014513