"git clean" fixes.

* en/clean-nested-with-ignored:
  dir: special case check for the possibility that pathspec is NULL
  clean: fix theoretical path corruption
  clean: rewrap overly long line
  clean: avoid removing untracked files in a nested git repository
  clean: disambiguate the definition of -d
  git-clean.txt: do not claim we will delete files with -n/--dry-run
  dir: add commentary explaining match_pathspec_item's return value
  dir: if our pathspec might match files under a dir, recurse into it
  dir: make the DO_MATCH_SUBMODULE code reusable for a non-submodule case
  dir: also check directories for matching pathspecs
  dir: fix off-by-one error in match_pathspec_item
  dir: fix typo in comment
  t7300: add testcases showing failure to clean specified pathspecs