fast-import.c: fix regression due to strbuf conversion

Without this strbuf_detach(), it yields a double free later, the
command is in fact stashed, and this is not a memory leak.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1616,6 +1616,7 @@ static void cmd_data(struct strbuf *sb)
 		char *term = xstrdup(command_buf.buf + 5 + 2);
 		size_t term_len = command_buf.len - 5 - 2;
 
+		strbuf_detach(&command_buf, NULL);
 		for (;;) {
 			if (strbuf_getline(&command_buf, stdin, '\n') == EOF)
 				die("EOF in data (terminator '%s' not found)", term);