core/unit: merge unit names after merging deps
Before:
systemd[1]: issue-24990.service: Dependency Before=n/a dropped, merged into issue-24990.service
After:
systemd[1]: issue-24990.service: Dependency Before=test1.service dropped, merged into issue-24990.service
(cherry picked from commit 1d0c81a05b1605a5fc3db44d5a157a1d6876eda9)