t3030-merge-recursive: test known breakage with empty work tree

Sometimes when working with a large repository it can be useful to try
out a merge and only check out conflicting files to disk (for example as
a speed optimization on a server).  Until v1.7.7-rc1~28^2~20
(merge-recursive: When we detect we can skip an update, actually skip
it, 2011-08-11), it was possible to do so with the following idiom:

	# Prepare a temporary index and empty work tree.
	GIT_INDEX_FILE="$PWD/tmp-$$-index" &&
	export GIT_INDEX_FILE &&
	GIT_WORK_TREE="$PWD/tmp-$$-work" &&
	export GIT_WORK_TREE &&
	mkdir "$GIT_WORK_TREE" &&

	# Convince the index that our side is on disk.
	git read-tree -i -m $ours &&
	git update-index --ignore-missing --refresh &&

	# Merge their side into our side.
	bases=$(git merge-base --all $ours $theirs) &&
	git merge-recursive $bases -- $ours $theirs &&
	tree=$(git write-tree)

Nowadays, that still works and the exit status is the same, but
merge-recursive produces a diagnostic if "our" side renamed a file:

	error: addinfo_cache failed for path 'dst'

Add a test to document this regression.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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