commit-graph: verify swapped zero/non-zero generation cases

commit-graph: verify swapped zero/non-zero generation cases

In verify_one_commit_graph(), we have code that complains when a commit
is found with a generation number of zero, and then later with a
non-zero number. It works like this:

  1. When we see an entry with generation zero, we set the
     generation_zero flag to GENERATION_ZERO_EXISTS.

  2. When we later see an entry with a non-zero generation, we complain
     if the flag is GENERATION_ZERO_EXISTS.

There's a matching GENERATION_NUMBER_EXISTS value, which in theory would
be used to find the case that we see the entries in the opposite order:

  1. When we see an entry with a non-zero generation, we set the
     generation_zero flag to GENERATION_NUMBER_EXISTS.

  2. When we later see an entry with a zero generation, we complain if
     the flag is GENERATION_NUMBER_EXISTS.

But that doesn't work; step 2 is implemented, but there is no step 1. We
never use NUMBER_EXISTS at all, and Coverity rightly complains that step
2 is dead code.

We can fix that by implementing that step 1.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>