sequencer: simplify handing of fixup with conflicts

sequencer: simplify handing of fixup with conflicts

Commit e032abd5a0 (rebase: fix rewritten list for failed pick,
2023-09-06) introduced an early return when res == -1, so if we enter
this conditional block then res is positive. After the last couple
of commits the only possible positive value is 1 so we can simplify
the code by removing the conditional call to intend_to_amend() and
call it error_with_patch() instead.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>