dashboard: more robust commit handler, and allow modifications
dashboard: more robust commit handler, and allow modifications

LGTM=rsc, adg
R=golang-codereviews, rsc, adg
CC=dvyukov, golang-codereviews
https://codereview.appspot.com/136540044

bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools