Fix the writing of invalid environment refs

Environment names that contained a space would cause an error
in GitLab 10.1 because a new guard clause was introduced in
Repository#write_ref to prevent such references from
existing. Use the slug instead to ensure that the name
is valid.

Closes #39182

app/models/environment.rb

@@ -110,7 +110,7 @@ class Environment < ActiveRecord::Base
   end
 
   def ref_path
-    "refs/#{Repository::REF_ENVIRONMENTS}/#{Shellwords.shellescape(name)}"
+    "refs/#{Repository::REF_ENVIRONMENTS}/#{generate_slug}"
   end
 
   def formatted_external_url

changelogs/unreleased/sh-fix-environment-write-ref.yml

+---
+title: Fix the writing of invalid environment refs
+merge_request:
+author:
+type: fixed

spec/models/environment_spec.rb

@@ -575,6 +575,16 @@ describe Environment do
     end
   end
 
+  describe '#ref_path' do
+    subject(:environment) do
+      create(:environment, name: 'staging / review-1')
+    end
+
+    it 'returns a path that uses the slug and does not have spaces' do
+      expect(environment.ref_path).to start_with('refs/environments/staging-review-1-')
+    end
+  end
+
   describe '#external_url_for' do
     let(:source_path) { 'source/file.html' }
     let(:sha) { RepoHelpers.sample_commit.id }