cmd-save: don't recurse into already-valid subdirs.

cmd-save: don't recurse into already-valid subdirs.

When iterating through the index, if we find out that a particular dir (like
/usr) has a known-valid sha1sum and isn't marked as changed, there's no need
to recurse into it at all.  This saves some pointless grinding through the
index when entire swaths of the tree are known to be already valid.