compat/snprintf: don't look at va_list twicejk/maint-snprintf-va-copy

If you define SNPRINTF_RETURNS_BOGUS, we use a special
git_vsnprintf wrapper assumes that vsnprintf returns "-1"
instead of the number of characters that you would need to
store the result.

To do this, it invokes vsnprintf multiple times, growing a
heap buffer until we have enough space to hold the result.
However, this means we evaluate the va_list parameter
multiple times, which is generally a bad thing (it may be
modified by calls to vsnprintf, yielding undefined
behavior).

Instead, we must va_copy it and hand the copy to vsnprintf,
so we always have a pristine va_list.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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