sequencer: use an enum to represent result of picking a commit

sequencer: use an enum to represent result of picking a commit

Rather than using an integer where -1 is an error, 0 is success and 1
indicates there were conflicts, use an enum. This is clearer and lets
us add a separate return value for commits that are dropped because
they become empty in the next commit.

Note we continue to use "return error(...)" to return errors and
take advantage of C's lax typing of enums

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