Adjust insufficient diff hunks being persisted on NoteDiffFileosw-ignore-diff-header-when-persisting-diff-hunk

This currently causes 500's errors when loading the MR page
(Discussion) in a few scenarios.

We were not considering detailed diff headers such as
"--- a/doc/update/mysql_to_postgresql.md\n+++ b/doc/update/mysql_to_postgresql.md"
to crop the diff. In order to address it, we're now using
Gitlab::Diff::Parser, clean the diffs and builds Gitlab::Diff::Line objects
we can iterate and filter on.

2 files changed, 10 insertions, 3 deletions

diff --git a/lib/gitlab/diff/file.rb b/lib/gitlab/diff/file.rb
index 765fb0289a8..2820293ad5c 100644
--- a/lib/gitlab/diff/file.rb
+++ b/lib/gitlab/diff/file.rb
@@ -78,9 +78,12 @@ module Gitlab
 
       # Returns the raw diff content up to the given line index
       def diff_hunk(diff_line)
-        # Adding 2 because of the @@ diff header and Enum#take should consider
-        # an extra line, because we're passing an index.
-        raw_diff.each_line.take(diff_line.index + 2).join
+        diff_line_index = diff_line.index
+        # @@ (match) header is not kept if it's found in the top of the file,
+        # therefore we should keep an extra line on this scenario.
+        diff_line_index += 1 unless diff_lines.first.match?
+
+        diff_lines.select { |line| line.index <= diff_line_index }.map(&:text).join("\n")
       end
 
       def old_sha
diff --git a/lib/gitlab/diff/line.rb b/lib/gitlab/diff/line.rb
index 0603141e441..a1e904cfef4 100644
--- a/lib/gitlab/diff/line.rb
+++ b/lib/gitlab/diff/line.rb
@@ -53,6 +53,10 @@ module Gitlab
         %w[match new-nonewline old-nonewline].include?(type)
       end
 
+      def match?
+        type == :match
+      end
+
       def discussable?
         !meta?
       end