The diff_opt infrastructure sets flags based on defaults and command
line options.  It is impossible to tell whether a flag has been set
as a default or on explicit request.  Update the structure so that
this detection is possible:

 * Add an extra "opt->touched_flags" that keeps track of all the
   fields that have been touched by DIFF_OPT_SET and DIFF_OPT_CLR.

 * You may continue setting the default values to the flags, like
   commands in the "log" family do in cmd_log_init_defaults(), but
   after you finished setting the defaults, you clear the
   touched_flags field;

 * And then you let the usual callchain call diff_opt_parse(),
   allowing the opt->flags be set or unset, while keeping track of
   which bits the user touched;

 * There is an optional callback "opt->set_default" that is called
   at the very beginning to let you inspect touched_flags and update
   opt->flags appropriately, before the remainder of the diffcore
   machinery is set up, taking the opt->flags value into account.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>