Fix pack.packSizeLimit and --max-pack-size handling

If the limit was sufficiently low, having a single object written
could bust the limit (by design), but caused the remaining allowed
size to go negative for subsequent objects, which for an unsigned
variable is a rather huge limit.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5300-pack-object.sh

@@ -375,4 +375,10 @@ test_expect_success 'index-pack with --strict' '
 	)
 '
 
+test_expect_success 'tolerate absurdly small packsizelimit' '
+	git config pack.packSizeLimit 2 &&
+	packname_9=$(git pack-objects test-9 <obj-list) &&
+	test $(wc -l <obj-list) = $(ls test-9-*.pack | wc -l)
+'
+
 test_done