SECURITY: Improve theme git import (#12694)

config/locales/server.en.yml

@@ -74,6 +74,7 @@ en:
       about_json_values: "about.json contains invalid values: %{errors}"
       modifier_values: "about.json modifiers contain invalid values: %{errors}"
       git: "Error cloning git repository, access is denied or repository is not found"
+      git_ref_not_found: "Unable to checkout git reference: %{ref}"
       unpack_failed: "Failed to unpack file"
       file_too_big: "The uncompressed file is too big."
       unknown_file_type: "The file you uploaded does not appear to be a valid Discourse theme."