_want_update_pkg: fix bug #523532

This fixes depth increment to handle _UNREACHABLE_DEPTH. Note that there
are some remaining "depth > deep" and "depth + 1" expressions that are
protected by short-circuit evaluation (when deep is True).

Fixes: 336ab90212c8 ("depgraph._add_dep: fix bug #520950")
X-Gentoo-Bug: 523532
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=523532