Fix Error 500 when creating a merge request that contains an image that was deleted and added

Steps to reproduce:

1. Start with a repo with an image
2. Add a commit to delete the image
3. Add another commit to replace the image with another image

In a diff comparison, we really just compare about what the image was before the diff, not
the direct parent of the last commit. This MR fixes that.

Closes #3893, gitlab-org/gitlab-ee#678

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

1 files changed, 8 insertions, 5 deletions

diff --git a/lib/gitlab/diff/file.rb b/lib/gitlab/diff/file.rb
index 5110bfbf898..c6bf25b5874 100644
--- a/lib/gitlab/diff/file.rb
+++ b/lib/gitlab/diff/file.rb
@@ -55,6 +55,12 @@ module Gitlab
         repository.commit(deleted_file ? old_ref : new_ref)
       end
 
+      def old_content_commit
+        return unless diff_refs
+
+        repository.commit(old_ref)
+      end
+
       def old_ref
         diff_refs.try(:base_sha)
       end
@@ -111,13 +117,10 @@ module Gitlab
         diff_lines.count(&:removed?)
       end
 
-      def old_blob(commit = content_commit)
+      def old_blob(commit = old_content_commit)
         return unless commit
 
-        parent_id = commit.parent_id
-        return unless parent_id
-
-        repository.blob_at(parent_id, old_path)
+        repository.blob_at(commit.id, old_path)
       end
 
       def blob(commit = content_commit)