config: values of pathname type can be prefixed with :(optional)

config: values of pathname type can be prefixed with :(optional)

Sometimes people want to specify additional configuration data
as "best effort" basis.  Maybe commit.template configuration file points
at somewhere in ~/template/ but on a particular system, the file may not
exist and the user may be OK without using the template in such a case.

When the value given to a configuration variable whose type is
pathname wants to signal such an optional file, it can be marked by
prepending ":(optional)" in front of it.  Such a setting that is
marked optional would avoid getting the command barf for a missing
file, as an optional configuration setting that names a missing
file is not even seen.

cf. <xmqq5ywehb69.fsf@gitster.g>

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>