t/t5318-commit-graph.sh: test generation zero transitions during fsck

The second test called "detect incorrect generation number" asserts that
we correctly warn during an fsck when we see a non-zero generation
number after seeing a zero beforehand.

The other transition (going from non-zero to zero) was previously
untested. Test both directions, and rename the existing test to make
clear which direction it is exercising.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5318-commit-graph.sh

@@ -450,14 +450,15 @@ GRAPH_BYTE_FANOUT2=$(($GRAPH_FANOUT_OFFSET + 4 * 255))
 GRAPH_OID_LOOKUP_OFFSET=$(($GRAPH_FANOUT_OFFSET + 4 * 256))
 GRAPH_BYTE_OID_LOOKUP_ORDER=$(($GRAPH_OID_LOOKUP_OFFSET + $HASH_LEN * 8))
 GRAPH_BYTE_OID_LOOKUP_MISSING=$(($GRAPH_OID_LOOKUP_OFFSET + $HASH_LEN * 4 + 10))
+GRAPH_COMMIT_DATA_WIDTH=$(($HASH_LEN + 16))
 GRAPH_COMMIT_DATA_OFFSET=$(($GRAPH_OID_LOOKUP_OFFSET + $HASH_LEN * $NUM_COMMITS))
 GRAPH_BYTE_COMMIT_TREE=$GRAPH_COMMIT_DATA_OFFSET
 GRAPH_BYTE_COMMIT_PARENT=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN))
 GRAPH_BYTE_COMMIT_EXTRA_PARENT=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 4))
 GRAPH_BYTE_COMMIT_WRONG_PARENT=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 3))
 GRAPH_BYTE_COMMIT_GENERATION=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 11))
+GRAPH_BYTE_COMMIT_GENERATION_LAST=$(($GRAPH_BYTE_COMMIT_GENERATION + $(($NUM_COMMITS - 1)) * $GRAPH_COMMIT_DATA_WIDTH))
 GRAPH_BYTE_COMMIT_DATE=$(($GRAPH_COMMIT_DATA_OFFSET + $HASH_LEN + 12))
-GRAPH_COMMIT_DATA_WIDTH=$(($HASH_LEN + 16))
 GRAPH_OCTOPUS_DATA_OFFSET=$(($GRAPH_COMMIT_DATA_OFFSET + \
 			     $GRAPH_COMMIT_DATA_WIDTH * $NUM_COMMITS))
 GRAPH_BYTE_OCTOPUS=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4))
@@ -596,11 +597,6 @@ test_expect_success 'detect incorrect generation number' '
 		"generation for commit"
 '
 
-test_expect_success 'detect incorrect generation number' '
-	corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_GENERATION "\01" \
-		"but zero elsewhere"
-'
-
 test_expect_success 'detect incorrect commit date' '
 	corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_DATE "\01" \
 		"commit date"
@@ -622,6 +618,16 @@ test_expect_success 'detect incorrect chunk count' '
 		$GRAPH_CHUNK_LOOKUP_OFFSET
 '
 
+test_expect_success 'detect mixed generation numbers (non-zero to zero)' '
+	corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_GENERATION_LAST "\0\0\0\0" \
+		"but non-zero elsewhere"
+'
+
+test_expect_success 'detect mixed generation numbers (zero to non-zero)' '
+	corrupt_graph_and_verify $GRAPH_BYTE_COMMIT_GENERATION "\0\0\0\0" \
+		"but zero elsewhere"
+'
+
 test_expect_success 'git fsck (checks commit-graph when config set to true)' '
 	git -C full fsck &&
 	corrupt_graph_and_verify $GRAPH_BYTE_FOOTER "\00" \