t5407: add a test demonstrating how interactive handles --skip differently

The post-rewrite hook is documented as being invoked by commands that
rewrite commits such as commit --amend and rebase, and that it will
be called for each rewritten commit.

Apparently, the three backends handled --skip'ed commits differently:
  am: treat the skipped commit as though it weren't rewritten
  merge: same as 'am' backend
  interactive: treat skipped commits as having been rewritten to empty
     (view them as an empty fixup to their parent)

For now, just add a testcase documenting the different behavior (use
--keep to force usage of the interactive machinery even though we have
no empty commits).  A subsequent commit will remove the inconsistency in
--skip handling.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5407-post-rewrite-hook.sh

@@ -125,6 +125,37 @@ test_expect_success 'git rebase -m --skip' '
 	verify_hook_input
 '
 
+test_expect_success 'git rebase with implicit use of interactive backend' '
+	git reset --hard D &&
+	clear_hook_input &&
+	test_must_fail git rebase --keep --onto A B &&
+	echo C > foo &&
+	git add foo &&
+	git rebase --continue &&
+	echo rebase >expected.args &&
+	cat >expected.data <<-EOF &&
+	$(git rev-parse C) $(git rev-parse HEAD^)
+	$(git rev-parse D) $(git rev-parse HEAD)
+	EOF
+	verify_hook_input
+'
+
+test_expect_success 'git rebase --skip with implicit use of interactive backend' '
+	git reset --hard D &&
+	clear_hook_input &&
+	test_must_fail git rebase --keep --onto A B &&
+	test_must_fail git rebase --skip &&
+	echo D > foo &&
+	git add foo &&
+	git rebase --continue &&
+	echo rebase >expected.args &&
+	cat >expected.data <<-EOF &&
+	$(git rev-parse C) $(git rev-parse HEAD^)
+	$(git rev-parse D) $(git rev-parse HEAD)
+	EOF
+	verify_hook_input
+'
+
 . "$TEST_DIRECTORY"/lib-rebase.sh
 
 set_fake_editor