Some old projects might be removed for some reason, especially
the downstream ones. It should be fine to just skip analysing them
because people have stopped working on them.
Test: run the script against an old release
Change-Id: I563905565c4c502036159fce6a386bba13ba25ea
Add a document explaining how to create a data studio dashboard using
the repo diff tool.
Renamed repo_diff_downstream.py to repo_diff_trees.py.
Renamed exclusions.txt to android_exclusions.txt.