so, this clearly isn't a very important bug, since it's been with us
since at least 9.4, which is the earliest tree i have readily at hand.
but recently i've found it annoying. i'm not completely wedded to it,
though (clearly, if i've only noticed it recently), so if other people
think the status quo is good, that's fine.
i must be fumble-fingering a lot these days, because i seem to be
accumulating unwanted buffers that i didn't use to notice. the
downside, of course, are the false positives: you do ":e newfile",
and realize you need to look at something (":e something"), and
when you go to switch back ("__" or ":e#") you find you have to
retype the ":e newfile" because it's gone. this is why i'd understand
if the feature's unwanted.
anyway... here's a patch that puts it back the way it used to be.