Re: [PATCH 2/2] fast-import: don't allow to note on empty branch

> 'reset' command makes fast-import start a branch from scratch. It's name
> is kept in lookup table but it's sha1 is null_sha1 (special value).
> 'notemodify' command can be used to add a note on branch head given it's
> name. lookup_branch() is used it that case and it doesn't check for
> null_sha1. So fast-import writes a note for null_sha1 object instead of
> giving a error.
>
> Add a check to deny adding a note on empty branch and add a corresponding
> test.