Backport MR create service changes from EE
Backport MR create service changes from EE

In EE, the `filter_params` method needs the MR to already have its
`target_project` assigned.

jirutka/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,dreampet/gitlab,dplarson/gitlabhq,dreampet/gitlab,jirutka/gitlabhq,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,dreampet/gitlab,dreampet/gitlab,dplarson/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,dplarson/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,t-zuehlsdorff/gitlabhq,mmkassem/gitlabhq