Bump github-linguist to 6.4.1
This resolves a Ruby 2.5-compatibility issue that was fixed in 6.3.0: https://github.com/github/linguist/pull/4195
The CommitLanguages RPC appears to still work despite throwing errors in
the Gitaly logs due to make_tmpname
no longer being present in Ruby
2.5.
May relate to #1570 (closed).
Edited by GitLab Release Tools Bot