Use tables and css to create the diffstat graph, fix scaling

There was no need to use image-files for the graphs, so lets drop them.
At the same time, fix scaling of the graphs so that the full width is
used only if atleast 100 LOC are changed in one of the files.

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

Makefile

@@ -60,7 +60,6 @@ install: all clean-cache
 	mkdir -p $(prefix)
 	install cgit $(prefix)/$(CGIT_SCRIPT_NAME)
 	install cgit.css $(prefix)/cgit.css
-	install add.png del.png $(prefix)/
 
 clean-cgit:
 	rm -f cgit *.o