sticky tags don't update for added/removed files

[Greg McGary]

Consider this scenario:

* check-out or update to a branch "foo".
* create a file, sticky tag is "foo".  Good.
* remove a file, sticky tag remains "foo".  Good.
* update to another branch tag with `-r bar', or back to trunk `-A'.
  The added file and the removed file still have "foo" as their sticky
  tags.  Bad.  (All other files have properly updated sticky tags)

It smells like a bug, but I thought I'd see if there was some subtle,
inscrutable reason why this behavior might be desirable.

If it's a bug, I volunteer to fix it, since I need it done within a
week.  8^)

Some possible wrinkles:

* removed file in old branch has different revision in new branch:
  should treat as conflict

* removed file in old branch has already been removed in new branch:
  seems like a noop, and the removal should be considered already
  complete.

* file added to old branch already exists in new branch:
  should treat as conflict (unless perhaps file content is identical,
  in which case the addition should be considered already complete?)

Greg