Merging MR: branch removal should be authored to the user who triggered the merge

Summary

Currently when PR is merged with Remove source branch enabled, the authorship of the source branch removal is currently set to the user who merged the MR. This is very confusing. It should show the user who merged the MR, not the one who originally submitted it.

Steps to reproduce

1. Open MR with user Foo from branch test-removal
2. Check the Remove source branch checkbox
3. Merge with user Bar

What is the current bug behavior?

Activity will show:

Foo deleted branch test-removal at Some / Project

What is the expected correct behavior?

Activity should show:

Bar deleted branch test-removal at Some / Project

GitLab 10.5.2.