t3310: illustrate failure to "notes merge --commit" inside $GIT_DIR/

The 'git notes merge' command expected to be run from the working
tree of the project being annotated, and did not anticipate getting
run inside $GIT_DIR/.

However, because we use $GIT_DIR/NOTES_MERGE_WORKTREE as a temporary
working space for the user to work on resolving conflicts, it is not
unreasonable for a user to run "git notes merge --commit" there. But
the command fails to do so.

Found-by: David Bremner <david@tethera.net>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3310-notes-merge-manual-resolve.sh

@@ -553,4 +553,23 @@ test_expect_success 'resolve situation by aborting the notes merge' '
 	verify_notes z
 '
 
+cat >expect_notes <<EOF
+foo
+bar
+EOF
+
+test_expect_failure 'switch cwd before committing notes merge' '
+	git notes add -m foo HEAD &&
+	git notes --ref=other add -m bar HEAD &&
+	test_must_fail git notes merge refs/notes/other &&
+	(
+		cd .git/NOTES_MERGE_WORKTREE &&
+		echo "foo" > $(git rev-parse HEAD) &&
+		echo "bar" >> $(git rev-parse HEAD) &&
+		git notes merge --commit
+	) &&
+	git notes show HEAD > actual_notes &&
+	test_cmp expect_notes actual_notes
+'
+
 test_done