Don't use File.join to join id branch path pairs as it is system dependent

Created by: cirosantilli

An id is the terminology for a branch path pair like:

master/path/to/file.md

It is not semantically correct to use File.join in those cases since that uses the separator of the current system, but the GitLab frontend is system agnostic, e.g., should in theory work in Windows where the path separator is \ without front-end changes.