git_config_set_multivar_in_file: all non-zero returns are errors

This function is just a thin wrapper for the "_gently" form of the function. But the gently form is designed to feed builtin/config.c, which passes our return code directly to its exit status, and thus uses positive error values for some cases. We check only negative values, meaning we would fail to die in some cases (e.g., a malformed key). This may or may not be triggerable in practice; we tend to use this non-gentle form only when setting internal variables, which would not have malformed keys. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>