config: add git_config_key_is_valid() for quiet validation

config: add git_config_key_is_valid() for quiet validation

Move the body of git_config_parse_key() into a static helper
do_parse_config_key() that takes a "quiet" flag and treats
store_key as optional.  git_config_parse_key() becomes a thin
wrapper.

Add git_config_key_is_valid() for callers that only need to
know whether a key is well-formed.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>