Ignore Signed-off-by in multi-line commit detection in CommitLinter

Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/gitlab/danger/commit_linter.rb

@@ -173,7 +173,7 @@ module Gitlab
       end
 
       def details
-        message_parts[2]
+        message_parts[2]&.gsub(/^Signed-off-by.*$/, '')
       end
 
       def line_too_long?(line)

spec/lib/gitlab/danger/commit_linter_spec.rb

@@ -86,6 +86,7 @@ describe Gitlab::Danger::CommitLinter do
       "A commit message" | false
       "A commit message\n" | false
       "A commit message\n\n" | false
+      "A commit message\n\nSigned-off-by: User Name <user@name.me>" | false
       "A commit message\n\nWith details" | true
     end