rebase -i: also avoid SHA-1 collisions with missingCommitsCheck

When `rebase.missingCommitsCheck` is in effect, we use the backup of the
todo list that was copied just before the user was allowed to edit it.

That backup is, of course, just as susceptible to the hash collision as
the todo list itself: a reworded commit could make a previously
unambiguous short commit ID ambiguous all of a sudden.

So let's not just copy the todo list, but let's instead write out the
backup with expanded commit IDs.

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

rebase-interactive.c

@@ -104,9 +104,11 @@ int edit_todo_list(struct repository *r, struct todo_list *todo_list,
 				    -1, flags | TODO_LIST_SHORTEN_IDS | TODO_LIST_APPEND_TODO_HELP))
 		return error_errno(_("could not write '%s'"), todo_file);
 
-	if (initial && copy_file(rebase_path_todo_backup(), todo_file, 0666))
-		return error(_("could not copy '%s' to '%s'."), todo_file,
-			     rebase_path_todo_backup());
+	if (initial &&
+	    todo_list_write_to_file(r, todo_list, rebase_path_todo_backup(),
+				    shortrevisions, shortonto, -1,
+				    (flags | TODO_LIST_APPEND_TODO_HELP) & ~TODO_LIST_SHORTEN_IDS) < 0)
+		return error(_("could not write '%s'."), rebase_path_todo_backup());
 
 	if (launch_sequence_editor(todo_file, &new_todo->buf, NULL))
 		return -2;

t/t3404-rebase-interactive.sh

@@ -1277,6 +1277,8 @@ test_expect_success SHA1 'short SHA-1 collide' '
 			.git/rebase-merge/git-rebase-todo.tmp &&
 		grep "^pick [0-9a-f]\{40\}" \
 			.git/rebase-merge/git-rebase-todo &&
+		grep "^pick [0-9a-f]\{40\}" \
+			.git/rebase-merge/git-rebase-todo.backup &&
 		git rebase --continue
 	) &&
 	collide2="$(git rev-parse HEAD~1 | cut -c 1-4)" &&