Add graphical diffstat to commit view

The diffstat is calculated against the leftmost parent of the commit. This
gives nice information for "normal" merges while octopus merges are less
than optimal, so the diffstat isn't calculated for those merges.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

Makefile

@@ -55,6 +55,7 @@ install: all clean-cache
 	mkdir -p $(prefix)
 	install cgit $(prefix)/cgit.cgi
 	install cgit.css $(prefix)/cgit.css
+	install add.png del.png $(prefix)/
 
 clean-cgit:
 	rm -f cgit *.o