compat: provide a fallback va_copy definition

compat: provide a fallback va_copy definition

va_copy is C99.  We have avoided using va_copy many times in the past,
which has led to a bunch of cut-and-paste.  From everything I found
searching the web, implementations have historically either provided
va_copy or just let your code assume that simple assignment of worked.

So my guess is that this will be sufficient, though we won't really
know for sure until somebody reports a problem.

Signed-off-by: Jeff King <peff@peff.net>
Improved-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>