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: NCharles Bailey <charles@hashpling.org>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/cat-file.c

@@ -504,6 +504,7 @@ int cmd_cat_file(int argc, const char **argv, const char *prefix)
 
 	git_config(git_cat_file_config, NULL);
 
+	batch.buffer_output = -1;
 	argc = parse_options(argc, argv, prefix, options, cat_file_usage, 0);
 
 	if (opt) {
@@ -527,6 +528,9 @@ int cmd_cat_file(int argc, const char **argv, const char *prefix)
 		usage_with_options(cat_file_usage, options);
 	}
 
+	if (batch.buffer_output < 0)
+		batch.buffer_output = batch.all_objects;
+
 	if (batch.enabled)
 		return batch_objects(&batch);