This bookmarklet is probably overkill and doesn't work with subtasks. Instead, just copy and paste the Asana task's link into your commit message and the relevant task will show up in Asana.
To quickly copy and paste your browser's current link: command-L then command-C. Works on any page ;-)
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Easily copy task titles for Github. Drag this link into your bookmarks to copy
task title #123456
(the task id) to your clipboard while in Asana.
Can't see the link? Go to http://bl.ocks.org/mikhuang/35b5ab17df3ec19e098f45f5fda643f5