t7001: add failure test which triggers assertion

`git mv a/a.txt a b/` is a nonsense instruction.  Instead of failing
gracefully the command trips over itself,[1] leaving behind unfinished
work:

1. first it moves `a/a.txt` to `b/a.txt`; then
2. tries to move `a/`, including `a/a.txt`; then
3. figures out that it’s in a bad state (assertion); and finally
4. aborts.

Now you’re left with a partially-updated index.

The command should instead fail gracefully and make no changes to the
index until it knows that it can complete a sensible action.

For now just add a failing test since this has been known about for
a while.[2]

† 1: Caused by a `pos >= 0` assertion
[2]: https://lore.kernel.org/git/d1f739fe-b28e-451f-9e01-3d2e24a0fe0d@app.fastmail.com/

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Author: LemmingAvalanche

t/t7001-mv.sh

@@ -551,4 +551,16 @@ test_expect_success 'moving nested submodules' '
 	git status
 '
 
+test_expect_failure 'nonsense mv triggers assertion failure and partially updated index' '
+	test_when_finished git reset --hard HEAD &&
+	git reset --hard HEAD &&
+	mkdir -p a &&
+	mkdir -p b &&
+	>a/a.txt &&
+	git add a/a.txt &&
+	test_must_fail git mv a/a.txt a b &&
+	git status --porcelain >actual &&
+	grep "^A[ ]*a/a.txt$" actual
+'
+
 test_done