git-remote: show all remotes with "git remote show"

git-remote: show all remotes with "git remote show"

Many other commands use the "no arguments" form to show a
list (e.g., git-branch, git-tag). While we did show all
remotes for just "git remote", we displayed a usage error
for "git remote show" with no arguments. This is
counterintuitive, since by giving it _more_ information, we
get _less_ result.

The usage model can now be thought of as:

  - "git remote show <remote>": show a remote
  - "git remote show": show all remotes
  - "git remote": assume "show"; i.e., shorthand for "git remote show"

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>