sequencer: be more careful with external merge

If an external merge strategy cannot merge (for example because it
would overwrite an untracked file) it exits with a non-zero exit
code other than 1. This should be treated differently from a merge
with conflicts, which is signaled by an exit code of 1, because, as
the merge failed, we need to reschedule the last pick. The caller
expects us to return -1 in this case. Also reschedule without trying
to merge if the commit message cannot be written as that prevents us
from successfully picking the commit.

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

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