Once a PB workspace is prepared for git you can use any git feature, even if PB does not support it internally. To do that, follow the steps described in "Add a workspace to git". This will export all source code files from your PBLs to a folder called "ws_objects" so git can track them.
From this point on, the only internal PB feature related to git you absolutely need to use is "Refresh", which updates the actual PBLs with the current source code from ws_objects. Everything else can be done with an external git client of your choice. I prefer the command line interface myself.
I would recommend not commiting PBL changes because they are binary files and will lead to conflicts every time you try to merge them. Only add and commit new PBLs and the actual source code files, use "Refresh" to update PBLs after switching branches etc.