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>

t/t5318-commit-graph.sh

@@ -811,4 +811,31 @@ test_expect_success 'set up and verify repo with generation data overflow chunk'
 
 graph_git_behavior 'generation data overflow chunk repo' repo left right
 
+test_expect_failure 'overflow during generation version upgrade' '
+	git init overflow-v2-upgrade &&
+	(
+		cd overflow-v2-upgrade &&
+
+		# This commit will have a date at two seconds past the Epoch,
+		# and a (v1) generation number of 1, since it is a root commit.
+		#
+		# The offset will then be computed as 1-2, which will underflow
+		# to 2^31, which is greater than the v2 offset small limit of
+		# 2^31-1.
+		#
+		# This is sufficient to need a large offset table for the v2
+		# generation numbers.
+		test_commit --date "@2 +0000" base &&
+		git repack -d &&
+
+		# Test that upgrading from generation v1 to v2 correctly
+		# produces the overflow table.
+		git -c commitGraph.generationVersion=1 commit-graph write &&
+		git -c commitGraph.generationVersion=2 commit-graph write \
+			--changed-paths &&
+
+		git rev-list --all
+	)
+'
+
 test_done