status/commit: make sure --porcelain is not affected by user-facing config

status/commit: make sure --porcelain is not affected by user-facing config

The recent addition of status.branch started affecting what is shown
when "git status --porcelain" is run by mistake.  Identify the
configuration items that should be ignored under "--porcelain"
option, introduce a "deferred config" mechanism to keep the values
read from the configuration, and decide what value to use only after
we read both from configuration and command line.

Signed-off-by: Junio C Hamano <gitster@pobox.com>