stash: prevent warning about null bytes in input

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>