commit-graph tests: split up corrupt_graph_and_verify()

Split up the corrupt_graph_and_verify() function added in
d9b9f8a6fd ("commit-graph: verify catches corrupt signature",
2018-06-27) into its logical components of setting up the test itself,
doing the corruption in a particular way with "dd", and then finally
testing that stderr is what we expect.

This allows for re-using everything except the now slimmer
corrupt_graph_and_verify() to corrupt the graph in a way that doesn't
involve inserting a given byte sequence at a given position,
e.g. truncating it entirely to a custom value.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5318-commit-graph.sh

@@ -366,6 +366,19 @@ GRAPH_OCTOPUS_DATA_OFFSET=$(($GRAPH_COMMIT_DATA_OFFSET + \
 GRAPH_BYTE_OCTOPUS=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4))
 GRAPH_BYTE_FOOTER=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4 * $NUM_OCTOPUS_EDGES))
 
+corrupt_graph_setup() {
+	cd "$TRASH_DIRECTORY/full" &&
+	test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
+	cp $objdir/info/commit-graph commit-graph-backup
+}
+
+corrupt_graph_verify() {
+	grepstr=$1
+	test_must_fail git commit-graph verify 2>test_err &&
+	grep -v "^+" test_err >err &&
+	test_i18ngrep "$grepstr" err
+}
+
 # usage: corrupt_graph_and_verify <position> <data> <string> [<zero_pos>]
 # Manipulates the commit-graph file at the position
 # by inserting the data, optionally zeroing the file
@@ -376,17 +389,14 @@ corrupt_graph_and_verify() {
 	pos=$1
 	data="${2:-\0}"
 	grepstr=$3
-	cd "$TRASH_DIRECTORY/full" &&
+	corrupt_graph_setup &&
 	orig_size=$(wc -c < $objdir/info/commit-graph) &&
 	zero_pos=${4:-${orig_size}} &&
-	test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
-	cp $objdir/info/commit-graph commit-graph-backup &&
 	printf "$data" | dd of="$objdir/info/commit-graph" bs=1 seek="$pos" conv=notrunc &&
 	dd of="$objdir/info/commit-graph" bs=1 seek="$zero_pos" count=0 &&
 	generate_zero_bytes $(($orig_size - $zero_pos)) >>"$objdir/info/commit-graph" &&
-	test_must_fail git commit-graph verify 2>test_err &&
+	corrupt_graph_verify "$grepstr"
-	grep -v "^+" test_err >err &&
+
-	test_i18ngrep "$grepstr" err
 }
 
 test_expect_success 'detect bad signature' '