acme: scroll a directory window when navigating if:

- the cursor is on the last line
        - the navigation would put the cursor over the tag of the following text

R=rsc
CC=smckean83
https://codereview.appspot.com/15280045

plumb/fileaddr

@@ -1,4 +1,4 @@
 addrelem='((#?[0-9]+)|(/[A-Za-z0-9_\^]+/?)|[.$])'
 addr=:($addrelem([,;+\-]$addrelem)*)
 
-twocolonaddr = :([0-9]+)[:.]([0-9]+)
+twocolonaddr = ([0-9]+)[:.]([0-9]+)