In emails-on-push message, indicate message that individual file diff is too large

Diff overflow limits apply to different cases:

1. The entire diff exceeds 100 files OR 5000 lines
2. One single file exceeds 100 KB

Example case 2: Let's say we have a diff of 50 files, most of them under 100
KB, but one of them over that limit. In this case, the idea of
gitlab-org/gitlab-ce!2705 is that GitLab should show the diff of the 49 files
but omit the last one.

app/views/notify/repository_push_email.html.haml

@@ -68,14 +68,17 @@
             - else
               %strong
                 = diff_file.new_path
-          %hr
-          - diff_commit = diff_file.deleted_file ? @message.diff_refs.first : @message.diff_refs.last
-          - blob = @message.project.repository.blob_for_diff(diff_commit, diff_file)
-          - if blob && blob.respond_to?(:text?) && blob_text_viewable?(blob)
-            %table.code.white
-              - diff_file.highlighted_diff_lines.each do |line|
-                = render "projects/diffs/line", {line: line, diff_file: diff_file, line_code: nil, plain: true}
+          - if diff_file.too_large?
+            The diff for this file was not included because it is too large.
           - else
-            No preview for this file type
+            %hr
+            - diff_commit = diff_file.deleted_file ? @message.diff_refs.first : @message.diff_refs.last
+            - blob = @message.project.repository.blob_for_diff(diff_commit, diff_file)
+            - if blob && blob.respond_to?(:text?) && blob_text_viewable?(blob)
+              %table.code.white
+                - diff_file.highlighted_diff_lines.each do |line|
+                  = render "projects/diffs/line", {line: line, diff_file: diff_file, line_code: nil, plain: true}
+            - else
+              No preview for this file type
           %br
 

app/views/notify/repository_push_email.text.haml

@@ -43,7 +43,10 @@
         - else
           = diff_file.new_path
         \=====================================
-        != diff_file.diff.diff
+        - if diff_file.too_large?
+          The diff for this file was not included because it is too large.
+        - else
+          != diff_file.diff.diff
 
   - if @message.target_url
     \