builtin/repack.c: reword comment around pack-objects flags

builtin/repack.c: reword comment around pack-objects flags

The comment in this block is meant to indicate that passing '--all',
'--reflog', and so on aren't necessary when repacking with the
'--geometric' option.

But, it has two problems: first, it is factually incorrect ('--all' is
*not* incompatible with '--stdin-packs' as the comment suggests);
second, it is quite focused on the geometric case for a block that is
guarding against it.

Reword this comment to address both issues.

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