Copy, not append/cat, something like:
#!/bin/sh
#
#CDBU=$(date +"%F_%H%M%S")
#BAK="../abu/$CDBU"
#cp .config "$BAK"
# or set aside the configdiff after it is generated, whatever
#make clean (dir/dist)
# i tend to the following rather than the previous, YMMV
# rm -rf bin build_dir tmp
#git pull
#./scripts/feeds update -a
#./scripts/feeds install -a
#./scripts/diffconfig.sh > configdiff
#cp configdiff .config
#make defconfig;make oldconfig
make a script and remove comment on the bits that work for you.