Add changes_count to the merge requests API

This returns the `real_size` of the MR's diff, which is a string indicating the
number of changes. If the diff overflows after _n_ files, the string will be
`$n+`.

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 67cecb6a7ad..aeb6001cf46 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -478,6 +478,10 @@ module API
       expose :subscribed do |merge_request, options|
         merge_request.subscribed?(options[:current_user], options[:project])
       end
+
+      expose :changes_count do |merge_request, _options|
+        merge_request.merge_request_diff.real_size
+      end
     end
 
     class MergeRequestChanges < MergeRequest