Backport Ci::PipelineTriggerService
Because this API: (in lib/api/triggers.rb
)
post ":id/(ref/:ref/)trigger/pipeline"
Diverged too much from CE to EE. This was discovered in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2527 and introduced in https://gitlab.com/gitlab-org/gitlab-ee/commit/35d0b9ff41eb0b11548b142829e7586d05feb722#note_35958159