git-bundle: assorted fixes

git-bundle: assorted fixes

This patch fixes issues mentioned by Junio, Nico and Simon:

- I forgot to convert the usage string when removing the "--" from
  the subcommands,
- a style fix in the bundle_header,
- use xread() instead of read(),
- use write_or_die() instead of write(),
- make the bundle header extensible,
- fail if the whitespace after a sha1 of a reference is missing,
- close() the fds passed to a subprocess,
- in verify_bundle(), do not use "rev-list --stdin", but rather
  pass the revs directly (avoiding a fork()),
- fix a corrupted comment in show_object(), and
- fix the size check in index_pack.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>