stash: prevent warning about null bytes in input

The `no_changes` function calls the `untracked_files` function through command substitution. `untracked_files` will return null bytes because it runs ls-files with the '-z' option. Bash since version 4.4 warns about these null bytes. As they are not required for the test that is being done, make sure `untracked_files` does not output null bytes when not required. This is achieved by adding a parameter to the `untracked_files` function to specify wither `-z` should be passed to ls-files or not. This warning is triggered when running git stash save -u resulting in two warnings: git-stash: line 43: warning: command substitution: ignored null byte in input Signed-off-by: Kevin Daudt <me@ikke.info> Signed-off-by: Junio C Hamano <gitster@pobox.com>