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>