pipeline of last commit is skipped if a commit in current set contains `[ci skip]

Summary

please see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15772#note_51358613 I made few commits in a tiny time gap. one of these commit contained a [ci skip] but the ci is skipped for last commit also which doesn't had [ci skip].

Steps to reproduce

1. create a commit with [ci skip]
2. create one more commit without [ci skip]
3. push the commits

What is the current bug behavior?

the ci is skipped.

What is the expected correct behavior?

CI should run since last commit doesn't contains [ci skip]

/cc @rymai @grzesiek