[dwm] patch: don't update prevtags if nothing changed

MODKEY-Tab is very nice for bouncing between tags, but sometimes I
re-select the current tag using MODKEY-<n> (where <n> is 1-9). Now
both seltags and prevtags contain the same set, meaning that
MODKEY-Tab does nothing useful.

This patch modifies the view() function so that it doesn't change
prevtags unless the new set of tags is different from the old.

I'm a new dwm user, so it's entirely possible that I've missed
something in the zen of it all - feedback appreciated.

dme.

dme.

-- 
David Edmondson, http://dme.org