help: use parse_config_key for man config
help: use parse_config_key for man config

The resulting code ends up about the same length, but it is
a little more self-explanatory. It now explicitly documents
and checks the pre-condition that the incoming var starts
with "man.", and drops the magic offset "4".

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Reviewed-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git