cat-file: default to --buffer when --batch-all-objects is used

cat-file: default to --buffer when --batch-all-objects is used

Traditionally cat-file's batch-mode does not do any output
buffering. The reason is that a caller may have pipes
connected to its input and output, and would want to use
cat-file interactively, getting output immediately for each
input it sends.

This may involve a lot of small write() calls, which can be
slow. So we introduced --buffer to improve this, but we
can't turn it on by default, as it would break the
interactive case above.

However, when --batch-all-objects is used, we do not read
stdin at all. We generate the output ourselves as quickly as
possible, and then exit. In this case buffering is a strict
win, and it is simply a hassle for the user to have to
remember to specify --buffer.

This patch makes --buffer the default when --batch-all-objects
is used. Specifying "--buffer" manually is still OK, and you
can even override it with "--no-buffer" if you're a
masochist (or debugging).

For some real numbers, running:

  git cat-file --batch-all-objects --batch-check='%(objectname)'

on torvalds/linux goes from:

  real    0m1.464s
  user    0m1.208s
  sys     0m0.252s

to:

  real    0m1.230s
  user    0m1.172s
  sys     0m0.056s

for a 16% speedup.

Suggested-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>