Cannot view MR with deleted source/target project/branch

https://sentry.gitlap.com/gitlab/gitlabcom/issues/30911/

NoMethodError: undefined method `repository' for nil:NilClass
  app/models/merge_request.rb:309:in `source_branch_head'
    source_project.repository.commit(source_branch_ref) if source_branch_ref
  lib/gitlab/conflict/file_collection.rb:10:in `initialize'
    @our_commit = merge_request.source_branch_head.raw.raw_commit
  app/models/merge_request.rb:973:in `new'
    @conflicts ||= Gitlab::Conflict::FileCollection.new(self)
  app/models/merge_request.rb:973:in `conflicts'
    @conflicts ||= Gitlab::Conflict::FileCollection.new(self)
  app/models/merge_request.rb:993:in `conflicts_can_be_resolved_in_ui?'
    conflicts.files.each(&:lines)
...
(179 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `repository' for nil:NilClass

ActionView::Template::Error: undefined method `repository' for nil:NilClass

https://sentry.gitlap.com/gitlab/gitlabcom/issues/30613/

NoMethodError: undefined method `raw' for nil:NilClass
  lib/gitlab/conflict/file_collection.rb:11:in `initialize'
    @their_commit = merge_request.target_branch_head.raw.raw_commit
  app/models/merge_request.rb:973:in `new'
    @conflicts ||= Gitlab::Conflict::FileCollection.new(self)
  app/models/merge_request.rb:973:in `conflicts'
    @conflicts ||= Gitlab::Conflict::FileCollection.new(self)
  app/models/merge_request.rb:993:in `conflicts_can_be_resolved_in_ui?'
    conflicts.files.each(&:lines)
  lib/gitlab/metrics/instrumentation.rb:156:in `block in conflicts_can_be_resolved_in_ui?'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
...
(178 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `raw' for nil:NilClass

ActionView::Template::Error: undefined method `raw' for nil:NilClass

Caused by the new http://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/projects/merge_requests/_show.html.haml#L19 call and some methods on the MergeRequestEntity not handling this situation well.

added Next Patch Release ~480950 ~901060 regression labels

changed milestone to %9.2

assigned to @DouweM

mentioned in merge request !11501 (merged)

closed via merge request !11501 (merged)

@DouweM This was reported here https://gitlab.com/gitlab-org/gitlab-ce/issues/32437#note_29945181 and https://gitlab.com/gitlab-org/gitlab-ce/issues/32479 before this sentry issue was escalated. It seems I could have linked this from the sentry issue to increase awareness. However this issue should also be linked to the regression issue.