Rename Ci::Commit in specs

This is second iteration of renaming Ci::Commit occurences, this time I try to rename all `let's` and `factories` to match what type of object is created:

1. Rename factories: `ci_[...]_commit_[...]` to `ci_[...]_pipeline_[...]`,
2. Rename all `let's(:[ci_]commit)` to `let(:pipeline)`,
3. Rename all occurrences of `[ci_]commit` to `pipeline` if Pipeline object is created

cc @grzesiek @DouweM 

See merge request !4463