replay: die descriptively when invalid commit-ish is given

Giving an invalid commit-ish to `--onto` 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 `set_up_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 Note that the `--advance` case won’t be triggered in practice because of the “argument to --advance must be a reference” check (see the previous test, and commit). † 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>