commit-graph: check mixed generation validation when loading chain file

commit-graph: check mixed generation validation when loading chain file

In read_commit_graph_one(), we call validate_mixed_generation_chain()
after loading the graph. Even though we don't check the return value,
this has the side effect of clearing the read_generation_data flag,
which is important when working with mixed generation numbers.

But doing this in load_commit_graph_chain_fd_st() makes more sense:

  1. We are calling it even when we did not load a chain at all, which
     is pointless (you cannot have mixed generations in a single file).

  2. For now, all callers load the graph via read_commit_graph_one().
     But the point of factoring out the open/load in the previous commit
     was to let "commit-graph verify" call them separately. So it needs
     to trigger this function as part of the load.

     Without this patch, the mixed-generation tests in t5324 would start
     failing on "git commit-graph verify" calls, once we switch to using
     a separate open/load call there.

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