rebase --root: fix amending root commit messages

The code path that triggered that "BUG" really does not want to run
without an explicit commit message. In the case where we want to amend a
commit message, we have an *implicit* commit message, though: the one of
the commit to amend. Therefore, this code path should not even be
entered.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -784,7 +784,7 @@ static int run_git_commit(const char *defmsg, struct replay_opts *opts,
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	const char *value;
-	if (flags & CREATE_ROOT_COMMIT) {
+	if ((flags & CREATE_ROOT_COMMIT) && !(flags & AMEND_MSG)) {
 		struct strbuf msg = STRBUF_INIT, script = STRBUF_INIT;
 		const char *author = is_rebase_i(opts) ?
 			read_author_ident(&script) : NULL;

t/t3404-rebase-interactive.sh

@@ -971,7 +971,7 @@ test_expect_success 'rebase -i --root fixup root commit' '
 	test 0 = $(git cat-file commit HEAD | grep -c ^parent\ )
 '
-test_expect_failure 'rebase -i --root reword root commit' '
+test_expect_success 'rebase -i --root reword root commit' '
 	test_when_finished "test_might_fail git rebase --abort" &&
 	git checkout -b reword-root-branch master &&
 	set_fake_editor &&