workingctx: use normal dirs() instead of dirstate.dirs()
The workingctx class was using dirstate.dirs() as it's implementation. The
sparse extension maintains a pruned down version of the dirstate, so this
resulted in the workingctx reporting an incorrect listing of directories
during merge calculations (it was detecting directory renames when it
shouldn't have).
The fix is to use the default implementation, which uses workingctx._manifest,
which unions the manifest with the dirstate to produce the correct overall
picture. This also produces more accurate output since it will no longer
return directories that have been entirely deleted in the dirstate.
Tests will be added to the sparse extension to detect regressions for this.