Simplify merge_ref_head methods

app/models/merge_request.rb

@@ -1080,16 +1080,14 @@ class MergeRequest < ApplicationRecord
 
   # Returns the current merge-ref HEAD commit.
   #
+  # Consider calling mergeability_check method _before_ this if you need
+  # the latest possible version of it (it's already automatically updated
+  # along the merge_status).
+  #
   def merge_ref_head
     project.repository.commit(merge_ref_path)
   end
 
-  # Returns the updated merge-ref HEAD commit.
-  #
-  def merge_ref_head!
-    merge_ref_head if check_mergeability.success?
-  end
-
   def ref_path
     "refs/#{Repository::REF_MERGE_REQUEST}/#{iid}/head"
   end

doc/api/merge_requests.md

@@ -1159,7 +1159,7 @@ Parameters:
 }
 ```
 
-## Return up to date merge-ref
+## Returns the up to date merge-ref HEAD commit
 
 Merge the changes between the merge request source and target branches into `refs/merge-requests/:iid/merge` 
 ref, of the target project repository, if possible. This ref will have the state the target branch would have if

lib/api/merge_requests.rb

@@ -398,11 +398,11 @@ module API
         present merge_request, with: Entities::MergeRequest, current_user: current_user, project: user_project
       end
 
-      desc 'Returns an up to date merge-ref HEAD'
+      desc 'Returns the up to date merge-ref HEAD commit'
       get ':id/merge_requests/:merge_request_iid/merge_ref' do
         merge_request = find_project_merge_request(params[:merge_request_iid])
 
-        result = merge_request.check_mergeability
+        result = ::MergeRequests::MergeabilityCheckService.new(merge_request).execute
 
         if result.success? && commit = merge_request.merge_ref_head
           present :commit_id, commit.sha