commit: allow -c/-C for all kinds of --fixup

commit: allow -c/-C for all kinds of --fixup

The previous commit allowed -m and -F for all --fixup variations.  The
-c/-C flags were blocked by the same higher-layer incompatibility check
that previously caught -F, namely die_for_incompatible_opt4() grouping
them with --fixup.

Drop --fixup from that check and route the resolved commit through
prepare_amend_commit() in the fixup path, mirroring the no-message-source
behaviour of --fixup=amend.  With this in place, -m/-F/-c/-C all behave
consistently across the plain, amend, and reword --fixup forms.

Signed-off-by: Erik Cervin-Edin <erik@cervined.in>
Signed-off-by: Junio C Hamano <gitster@pobox.com>