Allow "merge-recursive" to work in an empty (temporary) working
tree again when there are renames involved, correcting an old
regression in 1.7.7 era.

* bk/refresh-missing-ok-in-merge-recursive:
  merge-recursive.c: tolerate missing files while refreshing index
  read-cache.c: extend make_cache_entry refresh flag with options
  read-cache.c: refactor --ignore-missing implementation
  t3030-merge-recursive: test known breakage with empty work tree