Rev a1439e3...changes related to @auto and persistence

[Edward K. Ream]

I have spent all morning futzing with an annoying problem related to Shift-Ctrl-W (write-at-file-nodes).  After much experimentation I have an elegant solution.  This is now on the repo: please report any problems immediately.

The original problem was that the Shift-Ctrl-W didn't clear dirty bits in @auto trees, and also marked a previously-unchanged .leo file as marked!

It took awhile to realize that the @auto write code alters the @persistence tree, so the .leo file *should* be changed ;-)  There is no easy way for the persistence code to know whether the "before" and "after" versions of the persistence tree would actually be the same. After *much* experimentation with more complicated schemes I have found an easy way:

If *only* the persistence tree remains unchanged after the write-at-file-nodes command, Leo will automatically save the .leo file.  If there are *other* dirty nodes outside the @persistence tree, Leo will not presume to automatically save the .leo file.  This policy is safe, convenient and reasonable.  Leo users will mostly not even know the policy is in place, which is the best kind of policy.

Here is the checkin log for rev a1439e3...

QQQ
Added some tricky code to clear dirty bits in @auto trees after the write-at-file-nodes command.

An important Aha: the new code writes the .leo file (thereby writing changed persistence data), but *only* if there are no *other* changed nodes outside the persistence tree. Otherwise, neither the persistence data nor the still-unwritten dirty nodes are written, and the .leo file remains dirty.

This could be called a compromise, but it is much better than several other alternatives I tried today.
QQQ

Edward