Meta/cook: introduce -w HEAD as an ugly short-hand

This should not grok an arbitrary object name as "cook -w $name" is
supposed to be getting a filename, but it is handy to say "cook -w
HEAD" and get "git cat-file -p HEAD:whats-cooking.txt | cook -w -",
i.e. "which topics did I publicly threaten the list to merge soon?"
Meta/GRADUATED: still experimenting...
Meta/add-by: add them even to a message without any sign-off
What's cooking (2013/01 #08)
Meta/cook: hack to allow "Meta/cook -w HEAD~$n" to work
What's cooking (2013/01 #09)
Whats cooking (2013/01 #10)
Meta/cook: fix reading from the standard input stream