Prevent Compare page timout for large amount of commits

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

 v 6.6.0
   - Permissions: Developer now can manage issue tracker (modify any issue)
+  - Improve Code Compare page performance
 v 6.5.1
   - Fix branch selectbox when create merge request from fork

app/assets/stylesheets/generic/lists.scss

@@ -23,6 +23,12 @@
       }
     }
+    &.warning-row {
+      background-color: #fcf8e3;
+      border-color: #faebcc;
+      color: #8a6d3b;
+    }
+
     &.smoke { background-color: #f5f5f5; }
     &:hover {

app/views/projects/compare/show.html.haml

@@ -15,7 +15,14 @@
   %div.ui-box
     .title
       Commits (#{@commits.count})
-    %ul.well-list= render Commit.decorate(@commits), project: @project
+    - if @commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
+      %ul.well-list
+        - Commit.decorate(@commits.first(MergeRequestDiff::COMMITS_SAFE_SIZE)).each do |commit|
+          = render "projects/commits/inline_commit", commit: commit, project: @project
+        %li.warning-row.unstyled
+          other #{@commits.size - MergeRequestDiff::COMMITS_SAFE_SIZE} commits hidden to prevent performance issues.
+    - else
+      %ul.well-list= render Commit.decorate(@commits), project: @project
   - unless @diffs.empty?
     %h4 Diff