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>

usage.c

@@ -7,7 +7,7 @@
 
 static void report(const char *prefix, const char *err, va_list params)
 {
-	char msg[1024];
+	char msg[4096];
 	vsnprintf(msg, sizeof(msg), err, params);
 	fprintf(stderr, "%s%s\n", prefix, msg);
 }