Try to make reserved ref names more obvious

So that whenever we want to reserve more, we're aware,
and don't mess it up.

app/models/environment.rb

@@ -114,7 +114,7 @@ class Environment < ActiveRecord::Base
   end
 
   def ref_path
-    "refs/environments/#{Shellwords.shellescape(name)}"
+    "refs/#{Repository::REF_ENVIRONMENTS}/#{Shellwords.shellescape(name)}"
   end
 
   def formatted_external_url

app/models/merge_request.rb

@@ -797,7 +797,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def ref_path
-    "refs/merge-requests/#{iid}/head"
+    "refs/#{Repository::REF_MERGE_REQUEST}/#{iid}/head"
   end
 
   def ref_fetched?

app/models/repository.rb

 require 'securerandom'
 
 class Repository
+  REF_MERGE_REQUEST = 'merge-requests'
+  REF_KEEP_AROUND = 'keep-around'
+  REF_ENVIRONMENTS = 'environments'
+
+  RESERVED_REFS_NAMES = %W[
+    heads
+    tags
+    #{REF_ENVIRONMENTS}
+    #{REF_KEEP_AROUND}
+    #{REF_ENVIRONMENTS}
+  ].freeze
+
   include Gitlab::ShellAdapter
   include RepositoryMirroring
 
@@ -228,10 +240,10 @@ class Repository
     begin
       write_ref(keep_around_ref_name(sha), sha)
     rescue Rugged::ReferenceError => ex
-      Rails.logger.error "Unable to create keep-around reference for repository #{path}: #{ex}"
+      Rails.logger.error "Unable to create #{REF_KEEP_AROUND} reference for repository #{path}: #{ex}"
     rescue Rugged::OSError => ex
       raise unless ex.message =~ /Failed to create locked file/ && ex.message =~ /File exists/
-      Rails.logger.error "Unable to create keep-around reference for repository #{path}: #{ex}"
+      Rails.logger.error "Unable to create #{REF_KEEP_AROUND} reference for repository #{path}: #{ex}"
     end
   end
 
@@ -1152,7 +1164,7 @@ class Repository
   end
 
   def keep_around_ref_name(sha)
-    "refs/keep-around/#{sha}"
+    "refs/#{REF_KEEP_AROUND}/#{sha}"
   end
 
   def repository_event(event, tags = {})

app/services/projects/after_import_service.rb

 module Projects
   class AfterImportService
-    RESERVED_REFS_NAMES =
-      %w[heads tags merge-requests keep-around environments].freeze
-
     RESERVED_REFS_REGEXP =
-      %r{\Arefs/(?:#{Regexp.union(*RESERVED_REFS_NAMES)})/}
+      %r{\Arefs/(?:#{Regexp.union(*Repository::RESERVED_REFS_NAMES)})/}
 
     def initialize(project)
       @project = project

spec/services/projects/after_import_service_spec.rb

@@ -37,7 +37,7 @@ describe Projects::AfterImportService do
       end
     end
 
-    described_class::RESERVED_REFS_NAMES.each do |name|
+    Repository::RESERVED_REFS_NAMES.each do |name|
       context "with a ref in refs/#{name}/tmp" do
         before do
           repository.write_ref("refs/#{name}/tmp", sha)