Merge branch 'diff-parse-first-commit' into 'master'

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

Fixes gitlab_git#14

See merge request !2427

CHANGELOG

@@ -5,6 +5,7 @@ v 8.5.0 (unreleased)
 v 8.4.0 (unreleased)
   - Add pagination headers to already paginated API resources
+  - Properly generate diff of orphan commits, like the first commit in a repository
   - Improve the consistency of commit titles, branch names, tag names, issue/MR titles, on their respective project pages
   - 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)

lib/gitlab/diff/parser.rb

@@ -56,8 +56,9 @@ module Gitlab
       private
       def filename?(line)
-        line.start_with?('--- /dev/null', '+++ /dev/null', '--- a', '+++ b',
-                         '--- /tmp/diffy', '+++ /tmp/diffy')
+        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
       def identification_type(line)