Print notes number explained.

Printing just `[<number>]` was confusing - it looks like a ghost branch or something.
Add `note` (or `notes`, depending on number of notes) to give some details of what this number means.
Refactor code to follow DRY principle.

app/helpers/graph_helper.rb

@@ -7,7 +7,8 @@ module GraphHelper
     refs << commit_refs.join(' ')
 
     # append note count
-    refs << "[#{@graph.notes[commit.id]}]" if @graph.notes[commit.id] > 0
+    notes_count = @graph.notes[commit.id]
+    refs << "[#{notes_count} #{pluralize(notes_count, 'note')}]" if notes_count > 0
 
     refs
   end

changelogs/unreleased/bugfix-graph-friendly-notes-number.yml

+---
+title: Show notes number more user-friendly in the graph
+merge_request: 13949
+author: Vladislav Kaverin
+type: changed