gnus: 'active' file for nnml:* getting out of sync

[Giorgos Keramidas]

I have noticed with a relatively recent trunk copy that if I compress
nnml folders with 'G z' of if I compact the entire nnml server in the
server buffer, by typing '^', then close and reopen Gnus a few times,
then new messages are marked as already 'old' ('O' marker) even if they
just arrived in the particular nnml folder.

After a bit of digging around I found that '~/Mail/active' does not
match what nnml thinks is inside the folder.  If there any way to force
an update of the nnml active file to match the in-memory state of the
nnml folder?