sequencer: simplify handling of fixup with conflicts

sequencer: simplify handling 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. That
means we can simplify the code by removing the conditional call to
intend_to_amend() and have error_failed_squash() request that it is
called in error_with_patch() instead.

Signed-off-by: Phillip Wood <redacted>
Signed-off-by: Junio C Hamano <redacted>