checks: Move per-ref checks into ChangesAccess

Now that we have the ChangesAccess class which handles access checks for
a set of changes, let's also move the SingleChangeAccess checks in
there. Like this, we only have a single class which handles all checks.

lib/gitlab/checks/changes_access.rb

@@ -18,6 +18,10 @@ module Gitlab
       end
 
       def validate!
+        return if changes.empty?
+
+        single_access_checks!
+
         logger.log_timed("Running checks for #{changes.length} changes") do
           bulk_access_checks!
         end
@@ -27,6 +31,24 @@ module Gitlab
 
       protected
 
+      def single_access_checks!
+        # Iterate over all changes to find if user allowed all of them to be applied
+        changes.each.with_index do |change, index|
+          skip_lfs_integrity_check = index != 0
+
+          # If user does not have access to make at least one change, cancel all
+          # push by allowing the exception to bubble up
+          Checks::SingleChangeAccess.new(
+            change,
+            user_access: user_access,
+            project: project,
+            skip_lfs_integrity_check: skip_lfs_integrity_check,
+            protocol: protocol,
+            logger: logger
+          ).validate!
+        end
+      end
+
       def bulk_access_checks!
       end
     end

lib/gitlab/git_access.rb

@@ -339,22 +339,6 @@ module Gitlab
     end
 
     def check_access!
-      # Iterate over all changes to find if user allowed all of them to be applied
-      changes_list.each.with_index do |change, index|
-        skip_lfs_integrity_check = index != 0
-
-        # If user does not have access to make at least one change, cancel all
-        # push by allowing the exception to bubble up
-        Checks::SingleChangeAccess.new(
-          change,
-          user_access: user_access,
-          project: project,
-          skip_lfs_integrity_check: skip_lfs_integrity_check,
-          protocol: protocol,
-          logger: logger
-        ).validate!
-      end
-
       Checks::ChangesAccess.new(
         changes_list.changes,
         user_access: user_access,