diff: add test for --no-index executed outside repo

diff: add test for --no-index executed outside repo

470faf9 diff: move no-index detection to builtin/diff.c breaks the error
message for "git diff --no-index", when the command is executed outside
of a git repository and the wrong number of arguments are given. 6df5762
diff: don't read index when --no-index is given fixes the problem.

Add a test to guard against similar breakages in the future.

Signed-off-by: Thomas Gummerer <redacted>
Signed-off-by: Junio C Hamano <redacted>