update unseen sequence after display before this patch, if messages are displaied with a pager and during that time a new message comes to the folder. The new message is added to the unseen sequence. But after closing the pager the unseen sequence gets overwritten and the new message is removed from the unseen sequence. Now the sequence file gets reread before remove the shown messages from the unseen sequenece.
remove Previus-Sequence support The Previus-Sequence was racy and introduce a lot of write accesses. Also most use cases can solved better with shell history and command line editing.