@elextr requested changes on this pull request.
The space is currently only added to pre-0.19 formats, so adding it to all formats is an incompatible change. It assumes that the user is typing a full argument as they are required to do pre-0.19 but post 0.19 formats allow them to only have to type part of the argument.
Currently if the user configured "my_build_script f" in the GUI for custom command and typed "oo" the command is "my_build_script foo" but this change will make it "my_build_script f oo" with no way to recover.