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

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>