config: format paths gently

config: format paths gently

Move the logic for formatting path config values into a helper method
and use gentle parsing when needed.

We need to be careful about how to handle the ':(optional)' macro, which
as tested in t1311-config-optional.sh must allow for ignoring a missing
path when other multiple values exist, but cause 'git config get' to
fail if it is the only possible value and thus no result is output.

In the case of our list, we need to omit those values silently. This
necessitates the use of the 'gently' parameter here.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>