commit-graph: return generation from memory

The commit_graph_generation() method used to report a value of
GENERATION_NUMBER_INFINITY if the commit_graph_data_slab had an instance
for the given commit but the graph_pos indicated the commit was not in
the commit-graph file.

However, an upcoming change will introduce the ability to set generation
values in-memory without writing the commit-graph file. Thus, we can no
longer trust 'graph_pos' to indicate whether or not the generation
member can be trusted.

Instead, trust the 'generation' member if the commit has a value in the
slab _and_ the 'generation' member is non-zero. Otherwise, treat it as
GENERATION_NUMBER_INFINITY.

This only makes a difference for a very old case for the commit-graph:
the very first Git release to write commit-graph files wrote zeroes in
the topological level positions. If we are parsing a commit-graph with
all zeroes, those commits will now appear to have
GENERATION_NUMBER_INFINITY (as if they were not parsed from the
commit-graph).

I attempted several variations to work around the need for providing an
uninitialized 'generation' member, but this was the best one I found. It
does require a change to a verification test in t5318 because it reports
a different error than the one about non-zero generation numbers.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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