Merge branch 'jc/ls-files-killed-optim' into maint

"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