sequencer: don't say BUG on bogus input

When cherry-picking a single commit, we go through a special
code path that avoids creating a sequencer todo list at all.
This path expects our revision parsing to turn up exactly
one commit, and dies with a BUG if it doesn't.

But it's actually quite easy to fool. For example:

  $ git cherry-pick --author=no.such.person HEAD
  error: BUG: expected exactly one commit from walk
  fatal: cherry-pick failed

This isn't a bug; it's just bogus input.

The condition to trigger this message actually has two
parts:

  1. We saw no commits. That's the case in the example
     above. Let's drop the "BUG" here to make it clear that
     the input is the problem. And let's also use the phrase
     "empty commit set passed", which matches what we say
     when we do a real revision walk and it turns up empty.

  2. We saw more than one commit. That one _should_ be
     impossible to trigger, since we fed at most one tip and
     provided the no_walk option (and we'll have already
     expanded options like "--branches" that can turn into
     multiple tips). If this ever triggers, it's an
     indication that the conditional added by 7acaaac275
     (revert: allow single-pick in the middle of cherry-pick
     sequence, 2011-12-10) needs to more carefully define
     the single-pick case.

     So this can remain a bug, but we'll upgrade it to use
     the BUG() macro, which would make it easier to detect
     and analyze if it does trigger.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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