git-config: collect values instead of immediately printing

git-config: collect values instead of immediately printing

This is a refactor that will allow us to more easily tweak
the behavior for multi-valued variables, and it will
ultimately allow us to remove a lot git-config's custom code
in favor of the regular git_config code.

It does mean we're no longer streaming, and we're storing
more in memory for the --get-all case, but in practice it is
a tiny amount of data, and the results are instantaneous.

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