config: plumb --fixed-value into config API

config: plumb --fixed-value into config API

The git_config_set_multivar_in_file_gently() and related methods now
take a 'flags' bitfield, so add a new bit representing the --fixed-value
option from 'git config'. This alters the purpose of the value_pattern
parameter to be an exact string match. This requires some initialization
changes in git_config_set_multivar_in_file_gently() and a new strcmp()
call in the matches() method.

The new CONFIG_FLAGS_FIXED_VALUE flag is initialized in builtin/config.c
based on the --fixed-value option, and that needs to be updated in
several callers.

This patch only affects some of the modes of 'git config', and the rest
will be completed in the next change.

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