Prevent 500 error on Compare page if diff read timout happens

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

app/controllers/projects/compare_controller.rb

@@ -16,6 +16,11 @@ class Projects::CompareController < Projects::ApplicationController
     @refs_are_same = compare.same
     @line_notes    = []
 
+    if @diffs == [Gitlab::Git::Diff::BROKEN_DIFF]
+      @diffs = []
+      @timeout = true
+    end
+
     diff_line_count = Commit::diff_line_count(@diffs)
     @suppress_diff = Commit::diff_suppress?(@diffs, diff_line_count) && !params[:force_show_diff]
     @force_suppress_diff = Commit::diff_force_suppress?(@diffs, diff_line_count)

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

@@ -22,10 +22,14 @@
   - if @diffs.present?
     = render "projects/commits/diffs", diffs: @diffs, project: @project
   - elsif @commits.size > MergeRequestDiff::COMMITS_SAFE_SIZE
-    .alert.alert-warning
-      %p
-        %strong Warning! This comparison includes more than #{MergeRequestDiff::COMMITS_SAFE_SIZE} commits.
+    .bs-callout.bs-callout-danger
+      %h4 This comparison includes more than #{MergeRequestDiff::COMMITS_SAFE_SIZE} commits.
       %p To preserve performance the line diff is not shown.
+  - elsif @timeout
+    .bs-callout.bs-callout-danger
+      %h4 Diff for this comparison is extremely large.
+      %p Use command line to browse diff for this comparison.
+
 
 - else
   .light-well