Fixes race condition in project uploads

Originally picked as: 3fc0dbcbebcd470fe14f8b5fb7ad55dc3912402e. But
given the internals changed, this was changed now. The changelog was
removed too.

app/services/projects/gitlab_projects_import_service.rb

@@ -22,8 +22,11 @@ module Projects
     private
 
     def import_upload_path
-      @import_upload_path ||= Gitlab::ImportExport
-        .import_upload_path(filename: "#{params[:namespace_id]}-#{params[:path]}")
+      @import_upload_path ||= Gitlab::ImportExport.import_upload_path(filename: tmp_filename)
+    end
+
+    def tmp_filename
+      "#{SecureRandom.hex}_#{params[:path]}"
     end
 
     def file

lib/gitlab/import_export.rb

@@ -15,9 +15,7 @@ module Gitlab
     end
 
     def import_upload_path(filename:)
-      milliseconds = Process.clock_gettime(Process::CLOCK_REALTIME, :millisecond)
-
-      File.join(storage_path, 'uploads', "#{milliseconds}-#{filename}")
+      File.join(storage_path, 'uploads', filename)
     end
 
     def project_filename

spec/features/projects/import_export/import_file_spec.rb

@@ -31,6 +31,7 @@ feature 'Import/Export - project import integration test', js: true do
 
       expect(page).to have_content('GitLab project export')
       expect(URI.parse(current_url).query).to eq("namespace_id=#{namespace.id}&path=test-project-path")
+      expect(Gitlab::ImportExport).to receive(:import_upload_path).with(filename: /\A[0-9a-f]{32}_test_project_export\.tar\.gz\z/).and_call_original
 
       attach_file('file', file)