config: implement --fixed-value with --get*

config: implement --fixed-value with --get*

The config builtin does its own regex matching of values for the --get,
--get-all, and --get-regexp modes. Plumb the existing 'flags' parameter
to the get_value() method so we can initialize the value-pattern argument
as a fixed string instead of a regex pattern.

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