Added a comment

src/create.c

@@ -186,6 +186,8 @@ to_chars (int negative, uintmax_t value, size_t valsize,
 	{
 	  int negsub;
 	  uintmax_t sub = substitute (&negsub) & maxval;
+	  /* FIXME: This is the only place where GNU_FORMAT differs from
+             OLDGNU_FORMAT. Apart from this they are completely identical. */
 	  uintmax_t s = (negsub &= archive_format == GNU_FORMAT) ? - sub : sub;
 	  char subbuf[UINTMAX_STRSIZE_BOUND + 1];
 	  char *sub_string = STRINGIFY_BIGINT (s, subbuf + 1);

src/tar.h

@@ -253,7 +253,9 @@ enum archive_format
   USTAR_FORMAT,                 /* POSIX.1-1988 (ustar) format */
   POSIX_FORMAT,			/* POSIX.1-2001 format */
   STAR_FORMAT,                  /* Star format defined in 1994 */
-  GNU_FORMAT			/* POSIX format with GNU extensions */
+  GNU_FORMAT			/* Same as OLDGNU_FORMAT with one exception:
+                                   see FIXME note for to_chars() function
+                                   (create.c:189) */
 };
 
 /* Information about a sparse file.  */