Use Google's zopflipng to optimize png images

[PeterDaveHello]

Re-compress the png images losslessly via Google's zopflipng

 release/share/git-webui/webui/img/doc/log-commit.png | Bin 151760 -> 121167 bytes
 release/share/git-webui/webui/img/doc/log-tree.png   | Bin 101056 -> 79561 bytes
 release/share/git-webui/webui/img/doc/workspace.png  | Bin 118190 -> 97656 bytes
 release/share/git-webui/webui/img/git-icon.png       | Bin 2936 -> 1519 bytes
 release/share/git-webui/webui/img/git-logo.png       | Bin 18788 -> 9357 bytes
 src/share/git-webui/webui/img/doc/log-commit.png     | Bin 151760 -> 121167 bytes
 src/share/git-webui/webui/img/doc/log-tree.png       | Bin 101056 -> 79561 bytes
 src/share/git-webui/webui/img/doc/workspace.png      | Bin 118190 -> 97656 bytes
 src/share/git-webui/webui/img/git-icon.png           | Bin 2936 -> 1519 bytes
 src/share/git-webui/webui/img/git-logo.png           | Bin 18788 -> 9357 bytes