cat-file: split --batch input lines on whitespace

cat-file: split --batch input lines on whitespace

If we get an input line to --batch or --batch-check that
looks like "HEAD foo bar", we will currently feed the whole
thing to get_sha1(). This means that to use --batch-check
with `rev-list --objects`, one must pre-process the input,
like:

  git rev-list --objects HEAD |
  cut -d' ' -f1 |
  git cat-file --batch-check

Besides being more typing and slightly less efficient to
invoke `cut`, the result loses information: we no longer
know which path each object was found at.

This patch teaches cat-file to split input lines at the
first whitespace. Everything to the left of the whitespace
is considered an object name, and everything to the right is
made available as the %(reset) atom. So you can now do:

  git rev-list --objects HEAD |
  git cat-file --batch-check='%(objectsize) %(rest)'

to collect object sizes at particular paths.

Even if %(rest) is not used, we always do the whitespace
split (which means you can simply eliminate the `cut`
command from the first example above).

This whitespace split is backwards compatible for any
reasonable input. Object names cannot contain spaces, so any
input with spaces would have resulted in a "missing" line.
The only input hurt is if somebody really expected input of
the form "HEAD is a fine-looking ref!" to fail; it will now
parse HEAD, and make "is a fine-looking ref!" available as
%(rest).

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