stash: do not pass strbuf by value

stash: do not pass strbuf by value

save_untracked_files() takes its 'files' parameter as struct strbuf
by value.  Passing a strbuf by value copies the struct but shares
the underlying buffer between caller and callee, risking a dangling
pointer and double-free if the callee reallocates.

The function needs both the buffer and its length for
pipe_command(), so a plain const char * is not sufficient here.
Switch the parameter to struct strbuf * and update the caller to
pass a pointer.

Signed-off-by: Deveshi Dwivedi <deveshigurgaon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>