Properly generate diff of orphan commits, like the first commit in a repository

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.4.0 (unreleased)
+  - Properly generate diff of orphan commits, like the first commit in a repository
   - Autocomplete data is now always loaded, instead of when focusing a comment text area (Yorick Peterse)
   - Improved performance of finding issues for an entire group (Yorick Peterse)
   - Added custom application performance measuring system powered by InfluxDB (Yorick Peterse)

lib/gitlab/diff/parser.rb

@@ -56,7 +56,8 @@ module Gitlab
       private
 
       def filename?(line)
-        line.start_with?('--- /dev/null', '+++ /dev/null', '--- a', '+++ b',
+        line.start_with?( '--- /dev/null', '+++ /dev/null', '--- a', '+++ b',
+                          '+++ a', # The line will start with `+++ a` in the reverse diff of an orphan commit
                           '--- /tmp/diffy', '+++ /tmp/diffy')
       end