chdir to the displayed directory

This isn't necessary for the actual operation of ncdu,
but helps tools like screen/tmux open a new terminal in the currently
viewed directory, which is what people probably expect.

src/dirlist.c

@@ -189,6 +189,10 @@ static void dirlist_fixup() {
 void dirlist_open(struct dir *d) {
   dirlist_par = d;
 
+  /* not necessary for any ncdu functionality,
+   * but enables screen/tmux to work out our cwd */
+  chdir(getpath(dirlist_par));
+
   /* set the head of the list */
   head_real = head = d == NULL ? NULL : d->sub;