Make sure that cached values from links_and_reverse_links_dict are not mutable inside#826

Eijebong merged 1 commit intotaskcluster:maintaskcluster/taskgraph:mainfrom

Eijebong:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaEijebong/taskgraph:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaCopy head branch name to clipboard