show: add space between multiple items

show: add space between multiple items

When showing an annotated tag, "git show" will always
display the pointed-to object. However, it didn't separate
the two with whitespace, making it more difficult to notice
where the new object started. For example:

  $ git tag -m 'my message' foo
  $ git show foo
  tag foo
  Tagger: Jeff King <peff@peff.net>
  Date:   Fri Jul 17 18:46:25 2009 -0400

  my message
  commit 41cabf8fed2694ba33e01d64f9094f2fc5e5805a
  Author: Jeff King <peff@peff.net>
  Date:   Thu Jul 16 17:31:34 2009 -0400
  ...

This patch respects and sets the rev.shown_one member to
prepend a blank line before showing a second item. We use
this member of rev_info instead of a local flag, because the
log_tree_commit we call into for showing commits already
respects and sets that flag. Meaning that everything will be
spaced properly if you intermix commits and tags, like:

  $ git show v1.6.3 v1.6.2 HEAD

In that case, a single blank line will separate the first
tag, the commit it points to, the second tag, the commit
that one points to, and the final commit.

While we're at it, let's also support trees, so that even
something as crazy as

  $ git show HEAD^{tree} HEAD~1^{tree} HEAD

will also be spaced in an easy-to-read way. However, we
intentionally do _not_ insert blank lines for blobs, so
that specifying multiple blobs gives a strict concatenation.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>