gitbuilder should be able to build multiple projects

[n0ha]

Right now, when I want to build multiple projects, I need to have multiple gitbuilder instances, because there is only one build dir.

[apenwarr]

True. The correct fix for this is to actually have multiple gitbuilders, but then be able to merge the results into a single viewer, I think, so I'll leave this open in the hope that we actually produce this someday.