Re: [PATCH 0/2] replay: die descriptively when invalid commit-ish

>
> From: Kristoffer Haugsbakk <code@khaugsbakk.name>
>
> You get this error when you for example mistype the argument to
> `--onto`:
>
>     fatal: Replaying down to root commit is not supported yet!
>
> Consider that you might not know yourself that you have mistyped
> something; then this looks even more puzzling. You might have given a
> range like `main..topic` but the command says that it would need to
> replay down to the root commit.
>
> The only thing that’s happened though is that `NULL` has been
> interpreted in the wrong way.
>
> Let’s instead die immediately when the real error happens, in other
> words when we can’t find the commit for the given commit-ish.
>
> Also add more regression tests.

> Somewhat unrelated to this change—and caveat not a C programmer—I was
> confused by `determine_replay_mode`.  Most of the function deals with
> three cases:
>
>     if (onto_name) {
>     ...
>     } else if (*advance_name) {
>     ...
>     } else {
>     ...
>     }
>
> But I don’t get the `else` since you now need to provide either `--onto`
> or `--advance`.  And both require an argument.  So when can the
> fallthrough `else` happen?
>
> I thought that maybe the `else` was old code that predated `--onto` and
> `--advance` being mandatory. But that whole method and this check was
> added in the same commit, in 22d99f01 (replay: add --advance or
> 'cherry-pick' mode, 2023-11-24):
>
>     if (!onto_name && !advance_name) {
>             error(_("option --onto or --advance is mandatory"));
>
> But just ignore this section if I’m simply confused.