demonstrate git-commit --dry-run exit code behaviour

In particular, show that --short and --porcelain, while implying
--dry-run, do not return the same exit code as --dry-run. This is due to
the wt_status.commitable flag being set only when a long status is
requested.

No fix is provided here; with [1], it should be trivial to fix though -
just a matter of calling wt_status_mark_commitable().

[1] http://article.gmane.org/gmane.comp.version-control.git/242489

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7501-commit.sh

@@ -61,11 +61,47 @@ test_expect_success 'nothing to commit' '
 	test_must_fail git commit -m initial
 '
 
+test_expect_success '--dry-run fails with nothing to commit' '
+	test_must_fail git commit -m initial --dry-run
+'
+
+test_expect_success '--short fails with nothing to commit' '
+	test_must_fail git commit -m initial --short
+'
+
+test_expect_success '--porcelain fails with nothing to commit' '
+	test_must_fail git commit -m initial --porcelain
+'
+
+test_expect_success '--long fails with nothing to commit' '
+	test_must_fail git commit -m initial --long
+'
+
 test_expect_success 'setup: non-initial commit' '
 	echo bongo bongo bongo >file &&
 	git commit -m next -a
 '
 
+test_expect_success '--dry-run with stuff to commit returns ok' '
+	echo bongo bongo bongo >>file &&
+	git commit -m next -a --dry-run
+'
+
+test_expect_failure '--short with stuff to commit returns ok' '
+	echo bongo bongo bongo >>file &&
+	git commit -m next -a --short
+'
+
+test_expect_failure '--porcelain with stuff to commit returns ok' '
+	echo bongo bongo bongo >>file &&
+	git commit -m next -a --porcelain
+'
+
+test_expect_success '--long with stuff to commit returns ok' '
+	echo bongo bongo bongo >>file &&
+	git commit -m next -a --long
+'
+
 test_expect_success 'commit message from non-existing file' '
 	echo more bongo: bongo bongo bongo bongo >file &&
 	test_must_fail git commit -F gah -a