`git annex whereused` would report where in the git repository a
key is used, as a complement to `git-annex unused`.

Use cases include users not getting confused about why git-annex unused
says a key is used.

Also, it could scan through history to find where a key *was* used.
git-annex unused outputs a suggestion to use a rather hairy `git log -S`
command to do that currently.

If it does both these things, it could explain why git-annex unused
considers a key used despite a previous git rev referring to it. Eg:

	# git annex whereused SHA1--foo
	checking index... unused
	checking branches... unused
	checking tags... unused
	checking history... last used in master^40:somefile
	checking reflog... last used in HEAD@{30}:somefile

--[[Joey]]