unused .git objects end up in the initramfs image

cut scripts that pull in entire source trees should filter out `.git` objects.
fstests images are particularly bloated due to their presence, with an extra 28M+:
```
./rapido cut -B fstests-xfs && lsinitrd initrds/myinitrd |awk '/\/.git\// { totalbytes += $5 } END {print "got: " totalbytes}'
got: 29911893
```