Re: [PATCH v6 6/6] clean: teach clean -d to preserve ignored paths

> There is an implicit assumption that a directory containing only
> untracked and ignored paths should itself be considered untracked. This
> makes sense in use cases where we're asking if a directory should be
> added to the git database, but not when we're asking if a directory can
> be safely removed from the working tree; as a result, clean -d would
> assume that an "untracked" directory containing ignored paths could be
> deleted, even though doing so would also remove the ignored paths.
>
> To get around this, we teach clean -d to collect ignored paths and skip
> an untracked directory if it contained an ignored path, instead just
> removing the untracked contents thereof. To achieve this, cmd_clean()
> has to collect all untracked contents of untracked directories, in
> addition to all ignored paths, to determine which untracked dirs must be
> skipped (because they contain ignored paths) and which ones should *not*
> be skipped.
>
> For this purpose, correct_untracked_entries() is introduced to prune a
> given dir_struct of untracked entries containing ignored paths and those
> untracked entries encompassed by the untracked entries which are not
> pruned away.
>
> A memory leak is also fixed in cmd_clean().
>
> This also fixes the known breakage in t7300, since clean -d now skips
> untracked directories containing ignored paths.