dir: synchronize treat_leading_path() and read_directory_recursive()

Our optimization to avoid calling into read_directory_recursive() when all pathspecs have a common leading directory mean that we need to match the logic that read_directory_recursive() would use if we had just called it from the root. Since it does more than call treat_path() we need to copy that same logic. Alternatively, we could try to change treat_path to return path_recurse for an untracked directory under the given special circumstances that this logic checks for, but a simple switch results in many test failures such as 'git clean -d' not wiping out untracked but empty directories. To work around that, we'd need the caller of treat_path to check for path_recurse and sometimes special case it into path_untracked. In other words, we'd still have extra logic in both places. Needing to duplicate logic like this means it is guaranteed someone will eventually need to make further changes and forget to update both locations. It is tempting to just nuke the leading_directory special casing to avoid such bugs and simplify the code, but unpack_trees' verify_clean_subdirectory() also calls read_directory() and does so with a non-empty leading path, so I'm hesitant to try to restructure further. Add obnoxious warnings to treat_leading_path() and read_directory_recursive() to try to warn people of such problems. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>