If you copy a dirty linux dir as a tree of symlinks and then "make distclean" in
the copy, most of the .o files get skipped.
$ cd linux; make defconfig; make -j $(nproc); cd ..
$ cp -sfR $PWD/linux boing
$ cd boing
$ make distclean
$ find . -name '*.o' | wc
892 892 20017
Reproduced in commit gb19e8c684703 on x86-64 (devuan ascii).