Show at most 100 commits in the web UI

When rendering 1 commit takes over 5ms (on my laptop: around 9ms), it
saves seconds in rendering time to show 100 instead of 500 commits.

app/models/merge_request_diff.rb

@@ -17,7 +17,7 @@ class MergeRequestDiff < ActiveRecord::Base
   include Sortable
 
   # Prevent store of diff if commits amount more then 500
-  COMMITS_SAFE_SIZE = 500
+  COMMITS_SAFE_SIZE = 100
 
   belongs_to :merge_request