Add tests for rebase -i --root without --onto

Test for likely breakages in t3404, including successful reordering of
non-conflicting changes with a new root, correct preservation of commit
message and author in a root commit when it is squashed with the
sentinel, and presence of the sentinel following a conflicting
cherry-pick of a new root.

Remove test_must_fail for git rebase --root without --onto from t3412 as
this case will now be successfully handled by an implicit git rebase -i.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -755,4 +755,31 @@ test_expect_success 'rebase-i history with funny messages' '
 	test_cmp expect actual
 '
 
+test_expect_success 'rebase -i --root re-order and drop commits' '
+	git checkout E &&
+	FAKE_LINES="3 1 2 5" git rebase -i --root &&
+	test E = $(git cat-file commit HEAD | sed -ne \$p) &&
+	test B = $(git cat-file commit HEAD^ | sed -ne \$p) &&
+	test A = $(git cat-file commit HEAD^^ | sed -ne \$p) &&
+	test C = $(git cat-file commit HEAD^^^ | sed -ne \$p) &&
+	test 0 = $(git cat-file commit HEAD^^^ | grep -c ^parent\ )
+'
+
+test_expect_success 'rebase -i --root retain root commit author and message' '
+	git checkout A &&
+	echo B >file7 &&
+	git add file7 &&
+	GIT_AUTHOR_NAME="Twerp Snog" git commit -m "different author" &&
+	FAKE_LINES="2" git rebase -i --root &&
+	git cat-file commit HEAD | grep -q "^author Twerp Snog" &&
+	git cat-file commit HEAD | grep -q "^different author$"
+'
+
+test_expect_success 'rebase -i --root temporary sentinel commit' '
+	git checkout B &&
+	FAKE_LINES="2" test_must_fail git rebase -i --root &&
+	git cat-file commit HEAD | grep "^tree 4b825dc642cb" &&
+	git rebase --abort
+'
+
 test_done

t/t3412-rebase-root.sh

@@ -22,10 +22,6 @@ test_expect_success 'prepare repository' '
 	test_commit 4 B
 '
 
-test_expect_success 'rebase --root expects --onto' '
-	test_must_fail git rebase --root
-'
-
 test_expect_success 'setup pre-rebase hook' '
 	mkdir -p .git/hooks &&
 	cat >.git/hooks/pre-rebase <<EOF &&