Fix truncated usage messages
The usage messages for some commands (such as 'git diff-tree')
are truncated because they don't fit in a fixed buffer of
1024 bytes.
It would be tempting to eliminate the buffer and the problem once
and for all by doing the output in three steps, but doing so could
(according to commit
d048a96e) increase the likelyhood of messing
up the display.
So we just increase the size of the buffer.
Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>