git-config: use git_config_with_options

git-config: use git_config_with_options

The git-config command has always implemented its own file
lookup and parsing order. This was necessary because its
duplicate-entry handling did not match the way git's
internal callbacks worked. Now that this is no longer the
case, we are free to reuse the existing parsing code.

This saves us a few lines of code, but most importantly, it
means that the logic for which files are examined is
contained only in one place and cannot diverge.

Signed-off-by: Jeff King <peff@peff.net>