optimization: read stats once and remember it

for an initial run and new commits this speeds stagit up a bit:
on an initial run of sbase goes from about 4 seconds to 2.8 on my machine.

now we can't use statsbuf, so create the stats string ourselves, while at it
color the + and - using a style (can be disabled for the color-haters out
there ;)).

style.css

@@ -79,10 +79,12 @@ pre a.h {
 	color: #00a;
 }
 
+span.i,
 pre a.i {
 	color: #070;
 }
 
+span.d,
 pre a.d {
 	color: #e00;
 }