bring the test for symlink handling (well, ignoring) up through the actual update command tests.  not quite joy yet because java file apis before 7 remain unbelievably ridiculous.
give up, make the directory exist again so we can see the symlink and the test can pass.

note that this dangling symlink invisibility thing implies our subrepo walk code is probably at least a little technically wrong as well.  practically speaking, i fail to care because the only case it needs to handle specially is when a symlink could make a submodule "appear" where there isn't one... which a dangling symlink can't do anyway.