fixup! test-lib: add test_dir_is_empty()

- add "-n" to test expression

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
fixup! submodules: Add the lib-submodule-update.sh test library

- avoid non-portable "cp -a"
- use "diff -r" without "git " when comparing directory trees
- simplify retrieving the SHA-1 of sub1

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
fixup! rebase: add t3426 for submodule updates

- fix broken &&-chain