Merge Request edit: make breadcrumbs consistent

Change the breadcrumbs to "Project > Merge Requests > MR ID", where the
ID links to the view that displays the merge request.

app/views/projects/merge_requests/edit.html.haml

+- add_to_breadcrumbs _("Merge requests"), project_merge_requests_path(@project)
+- breadcrumb_title @merge_request.to_reference
 - page_title _("Edit"), "#{@merge_request.title} (#{@merge_request.to_reference}", _("Merge requests")
 
 %h3.page-title

changelogs/unreleased/santigl-edit-mr-breadcrumbs.yml

+---
+title: 'Merge Request edit: make breadcrumbs consistent'
+merge_request: 62138
+author: Santiago Gil @santigl
+type: other