Fix uses of context->changed as a counter.

Fix uses of context->changed as a counter.

The first was in mx_update_tables(), but only when "not committing".
This is used by mh/maildir during an "occult" update, and in imap when
expunging the mailbox.  It meant to simply turn on changed when a
single changed header is seen.

The second use was in imap_sync_message_for_copy().  Previously this
was used for a server side copy/save, but is now also used for
fast-trash copying.  Remove the code that was trying to decrement the
counter: this function is not capable of properly setting a status
bit.