There is no openacs-5-0 branch. It's called oacs-5-0.
The tags and branches all used to be oacs instead of openacs until I screwed up and started using openacs in the 5.0 alphas. Now we have a fair number of both tags but the openacs- tags are more recent, so I'm just going to keep using openacs- unless someone complains. Which means that we'll branch 5.1 as openacs-5-1.