"git ls-files -k" needs to crawl only the part of the working tree
that may overlap the paths in the index to find killed files, but
shared code with the logic to find all the untracked files, which
made it unnecessarily inefficient.

* jc/ls-files-killed-optim:
  dir.c::test_one_path(): work around directory_exists_in_index_icase() breakage
  t3010: update to demonstrate "ls-files -k" optimization pitfalls
  ls-files -k: a directory only can be killed if the index has a non-directory
  dir.c: use the cache_* macro to access the current index