Add link to gitaly converation

This endpoint still has to be migrated, and this comment makes sure
everyone knows we are aware of this one.

[ci skip]

lib/gitlab/git/blob.rb

@@ -49,6 +49,7 @@ module Gitlab
         # Keep in mind that this method may allocate a lot of memory. It is up
         # to the caller to limit the number of blobs and blob_size_limit.
         #
+        # Gitaly migration issue: https://gitlab.com/gitlab-org/gitaly/issues/798
         def batch(repository, blob_references, blob_size_limit: nil)
           blob_size_limit ||= MAX_DATA_DISPLAY_SIZE
           blob_references.map do |sha, path|