Don't delete unmerged branches in UpdateRemoteMirror
An old bug was reintroduced in UpdateRemoteMirror while it was being ported to Go. The Ruby implementation did not delete branches in the remote that were not present in the local repository if they contained work not merged in the local repository's default branch. This MR brings the Go port in line with the previous behavior in the Ruby implementation and adds a regression test for the bug.
Closes #3760 (closed)
Edited by Sami Hiltunen