config: format expiry dates quietly

config: format expiry dates quietly

Move the logic for formatting expiry date config values into a helper
method and use quiet parsing when needed.

Note that git_config_expiry_date() will show an error on a bad parse and
not die() like most other git_config...() parsers. Thus, we use
'quietly' here instead of 'gently'.

There is an unfortunate asymmetry in these two parsing methods, but we
need to treat a positive response from parse_expiry_date() as an error
or we will get incorrect values.

This updates the behavior of 'git config list --type=expiry-date' to be
quiet when attempting parsing on non-date values.

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