After a `git-annex forget`, syncing with a remote that still has the
forgotten data in its git-annex branch merges that data right back in.

Reproduction script:

	chmod -R +w bench 2>/dev/null
	rm -rf bench 2>/dev/null
	mkdir bench
	cd bench
	git init a
	cd a
	git-annex init
	touch foo
	git-annex add foo
	git commit -m add
	cd ..
	git clone a b
	cd b
	git-annex get
	git-annex drop
	git-annex get
	git-annex drop
	cd ../a
	git-annex sync
	git-annex forget --force
	git-annex sync
	git-annex log foo
	cd ../b
	git-annex sync
	cd ../a
	git-annex log foo

--[[Joey]]

> [[fixed|done]] --[[Joey]]