Makefile: update "make clean" target after f91e5630ce7e
# should print nothing# should failabort: path 'b/x' is inside repo 'b'abort: path 'b/x' is inside repo 'b'# should failabort: path 'b/x' is inside repo 'b'# should arguably print nothing# should failabort: path 'b/a' is inside repo 'b'