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>

git-stash.sh

@@ -43,9 +43,16 @@ no_changes () {
 }
 
 untracked_files () {
+	if test "$1" = "-z"
+	then
+		shift
+		z=-z
+	else
+		z=
+	fi
 	excl_opt=--exclude-standard
 	test "$untracked" = "all" && excl_opt=
-	git ls-files -o -z $excl_opt -- "$@"
+	git ls-files -o $z $excl_opt -- "$@"
 }
 
 clear_stash () {
@@ -114,7 +121,7 @@ create_stash () {
 		# Untracked files are stored by themselves in a parentless commit, for
 		# ease of unpacking later.
 		u_commit=$(
-			untracked_files "$@" | (
+			untracked_files -z "$@" | (
 				GIT_INDEX_FILE="$TMPindex" &&
 				export GIT_INDEX_FILE &&
 				rm -f "$TMPindex" &&