vcs-svn/svndiff.c: squelch false "unused" warning from gcc

Curiously, pre_len given to read_length() does not trigger the same warning
even though the code structure is the same. Most likely this is because
read_offset() is used only once and inlining it will make gcc realize that
it has a chance to do more flow analysis. Alas, the analysis is flawed, so
it does not help X-<.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

vcs-svn/svndiff.c

@@ -295,7 +295,7 @@ int svndiff0_apply(struct line_buffer *delta, off_t delta_len,
 	if (read_magic(delta, &delta_len))
 		return -1;
 	while (delta_len) {	/* For each window: */
-		off_t pre_off;
+		off_t pre_off = pre_off; /* stupid GCC... */
 		size_t pre_len;
 
 		if (read_offset(delta, &pre_off, &delta_len) ||