When ce MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26144
was merged in, it caused failures in ee specs
https://gitlab.com/gitlab-org/gitlab-ee/issues/10567.

The fix for these specs is to select an "author" of the MR by first
filtering merge commits from the commit collection. This mirrors the
behaviour of MergeRequest#authors.

Previously, this test passed as MergeRequest#commits would always return
commits that lacked parent_ids, as they are initialized from data
kept in MergeRequestDiffCommit which does not have parent_ids data. This
meant that all MR commits were considered not to be merge commits, even
though in gitaly some did have parent ids.

However, now, CommitCollection#enrich! is called
CommitCollection#without_merge_commits is called (as it is when
MergeRequest#authors is called), so commit data is fetched from gitaly
and now have parent_ids data. This cause the tests to fail, because the
"author" being chosen from the merge request commits was in fact the
user who had committed a merge commit.