The other config-writing functions (git_config_set and
git_config_set_multivar) each have an -"in_file" version to
write a specific file. Let's add one for rename_section,
with the eventual goal of moving away from the magic
config_exclusive_filename global.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>