Update logic for validates_merge_request for tree of MR

For case:

Merge request №1: branch "feature" to "release" and remove "feature" branch

Merge request №2: branch "release" to "master" and remove "release" branch

Merge request №1 - broken :(
Merge branch 'patch-2' of https://github.com/zzet/gitlabhq into zzet-patch-2

Conflicts:
	app/controllers/projects/merge_requests_controller.rb