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>