cat-file: only split on whitespace when %(rest) is used

Commit c334b87b (cat-file: split --batch input lines on whitespace,
2013-07-11) taught `cat-file --batch-check` to split input lines on
the first whitespace, and stash everything after the first token
into the %(rest) output format element.  It claimed:

   Object names cannot contain spaces, so any input with
   spaces would have resulted in a "missing" line.

But that is not correct.  Refs, object sha1s, and various peeling
suffixes cannot contain spaces, but some object names can. In
particular:

  1. Tree paths like "[<tree>]:path with whitespace"

  2. Reflog specifications like "@{2 days ago}"

  3. Commit searches like "rev^{/grep me}" or ":/grep me"

To remain backwards compatible, we cannot split on whitespace by
default, hence we will ship 1.8.4 with the commit reverted.

Resurrect its attempt but in a weaker form; only do the splitting
when "%(rest)" is used in the output format. Since that element did
not exist at all before c334b87, old scripts cannot be affected.

The existence of object names with spaces does mean that you
cannot reliably do:

  echo ":path with space and other data" |
  git cat-file --batch-check="%(objectname) %(rest)"

as it would split the path and feed only ":path" to get_sha1. But
that command is nonsensical. If you wanted to see "and other data"
in "%(rest)", git cannot possibly know where the filename ends and
the "rest" begins.

It might be more robust to have something like "-z" to separate the
input elements. But this patch is still a reasonable step before
having that.  It makes the easy cases easy; people who do not care
about %(rest) do not have to consider it, and the %(rest) code
handles the spaces and newlines of "rev-list --objects" correctly.

Hard cases remain hard but possible (if you might get whitespace in
your input, you do not get to use %(rest) and must split and join
the output yourself using more flexible tools). And most
importantly, it does not preclude us from having different splitting
rules later if a "-z" (or similar) option is added.  So we can make
the hard cases easier later, if we choose to.

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