replay: die descriptively when invalid commit-ish

replay: die descriptively when invalid commit-ish

Giving an invalid commit-ish to `--onto` or `--advance` makes
git-replay(1) fail with:

    fatal: Replaying down to root commit is not supported yet!

Going backwards from this point:

1. `onto` is `NULL` from `determine_replay_mode`;
2. that function in turn calls `peel_committish`; and
3. here we return `NULL` if `repo_get_oid` fails.

Let’s die immediately with a descriptive error message instead.

Doing this also provides us with a descriptive error if we “forget” to
provide an argument to `--onto` (but we really do unintentionally):[1]

    $ git replay --onto ^main topic1
    fatal: '^main' is not a valid commit-ish

† 1: The argument to `--onto` is mandatory and the option parser accepts
     both `--onto=<name>` (stuck form) and `--onto name`. The latter
     form makes it easy to unintentionally pass something to the option
     when you really meant to pass a positional argument.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>