gitweb: Optimize git-favicon.png

Reduce size of git-favicon.png using a combination of optipng and
pngout. From 164 bytes to 115 bytes (30% reduction). Also reduce
git-logo.png's size by one byte using advcomp.

Signed-off-by: Benjamin Kramer <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: d0k