Parse conflict lines with CRLF endings correctly

What does this MR do and why?

When lines have CRLF endings, GitLab is not able to parse them correctly as we match the conflict markers after we delete the newline character. That only deletes the LF and keep the CR.

To fix it, we also delete the CR before we match the conflict markers.

How to set up and validate locally

Follow the steps in #365343 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #365343 (closed)

lib/gitlab/git/conflict/parser.rb

@@ -27,7 +27,7 @@ def parse(text, our_path:, their_path:, parent_file: nil)
             conflict_end = ">>>>>>> #{their_path}"
 
             text.each_line.map do |line|
-              full_line = line.delete("\n")
+              full_line = line.chomp
 
               if full_line == conflict_start
                 validate_delimiter!(type.nil?)