t5318: demonstrate commit-graph generation v2 corruption

When upgrading a commit-graph using generation v1 to one using
generation v2, it is possible to force Git into a corrupt state where it
(incorrectly) believes that a GDO2 chunk is necessary, *after* deciding
not to write one.

This makes subsequent reads using the commit-graph produce the following
error message:

    fatal: commit-graph requires overflow generation data but has none

Demonstrate this bug by increasing our test coverage to include a
minimal example of upgrading a commit-graph from generation v1 to v2.
The only notable components of this test are:

  - The committer date of the commit is chosen carefully so that the
    offset underflows when computed using a v1 generation number, but
    would not overflow when using v2 generation numbers.

  - The upgrade to generation number v2 must read in the v1 generation
    numbers, which we can do by passing `--changed-paths`, which will
    force the commit-graph internals to call `fill_commit_graph_info()`.

A future patch will squash this bug.

Reported-by: Jeff King <peff@peff.net>
Reproduced-by: Will Chandler <wfc@wfchandler.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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