sequencer: finish parsing the todo list despite an invalid first line

Before the todo list is edited it is rewritten to shorten the OIDs of
the commits being picked and to append advice about editing the list.
The exact advice depends on whether the todo list is being edited for
the first time or not. After the todo list has been edited it is
rewritten to lengthen the OIDs of the commits being picked and to remove
the advice. If the edited list cannot be parsed then this last step is
skipped.

Prior to db81e50724 (rebase-interactive: use todo_list_write_to_file()
in edit_todo_list(), 2019-03-05) if the existing todo list could not be
parsed then the initial rewrite was skipped as well. This had the
unfortunate consequence that if the list could not be parsed after the
initial edit the advice given to the user was wrong when they re-edited
the list. This change relied on todo_list_parse_insn_buffer() returning
the whole todo list even when it cannot be parsed. Unfortunately if the
list starts with a "fixup" command then it will be truncated and the
remaining lines are lost. Fix this by continuing to parse after an
initial "fixup" commit as we do when we see any other invalid line.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
[jc: removed an apparently unneeded subshell around the test body]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -2680,7 +2680,7 @@ int todo_list_parse_insn_buffer(struct repository *r, char *buf,
 		if (fixup_okay)
 			; /* do nothing */
 		else if (is_fixup(item->command))
-			return error(_("cannot '%s' without a previous commit"),
+			res = error(_("cannot '%s' without a previous commit"),
 				command_to_string(item->command));
 		else if (!is_noop(item->command))
 			fixup_okay = 1;

t/t3404-rebase-interactive.sh

@@ -1596,6 +1596,32 @@ test_expect_success 'static check of bad command' '
 	test C = $(git cat-file commit HEAD^ | sed -ne \$p)
 '
 
+test_expect_success 'the first command cannot be a fixup' '
+	rebase_setup_and_clean fixup-first &&
+
+	cat >orig <<-EOF &&
+	fixup $(git log -1 --format="%h %s" B)
+	pick $(git log -1 --format="%h %s" C)
+	EOF
+
+	(
+		set_replace_editor orig &&
+		test_must_fail git rebase -i A 2>actual
+	) &&
+	grep "cannot .fixup. without a previous commit" actual &&
+	grep "You can fix this with .git rebase --edit-todo.." actual &&
+	# verify that the todo list has not been truncated
+	grep -v "^#" .git/rebase-merge/git-rebase-todo >actual &&
+	test_cmp orig actual &&
+
+	test_must_fail git rebase --edit-todo 2>actual &&
+	grep "cannot .fixup. without a previous commit" actual &&
+	grep "You can fix this with .git rebase --edit-todo.." actual &&
+	# verify that the todo list has not been truncated
+	grep -v "^#" .git/rebase-merge/git-rebase-todo >actual &&
+	test_cmp orig actual
+'
+
 test_expect_success 'tabs and spaces are accepted in the todolist' '
 	rebase_setup_and_clean indented-comment &&
 	write_script add-indent.sh <<-\EOF &&