Fix conflict resolving not updating merge requests
Previously the Go implementation of ResolveConflicts
updated the ref of a merge request without pushing the changes to the Git hooks. As a result, GitLab Rails had no idea a branch had been modified, so it could not refresh associated merge requests as a result.
To support this, we need to centralize the functionality in update_with_hooks.go
so that it can be used by the conflicts package.
Relates to gitlab#18917 (closed)
Edited by Stan Hu