pack-objects: use BUG(...) not die("BUG: ...")

Change this code added in da93d12 (pack-objects: be incredibly
anal about stdio semantics, 2006-04-02) to use BUG() instead.

See 1a07e59 (Update messages in preparation for i18n, 2018-07-21)
for when the "BUG: " prefix was added, and [1] for background on the
Solaris behavior that prompted the exhaustive error checking in this
fgets() loop.

1. https://lore.kernel.org/git/[email protected]/

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

builtin/pack-objects.c

@@ -3397,7 +3397,7 @@ static void read_object_list_from_stdin(void)
 			if (feof(stdin))
 				break;
 			if (!ferror(stdin))
-				die("BUG: fgets returned NULL, not EOF, not error!");
+				BUG("fgets returned NULL, not EOF, not error!");
 			if (errno != EINTR)
 				die_errno("fgets");
 			clearerr(stdin);