Rationalize output selection in rev-parse.

Rationalize output selection in rev-parse.

Earlier rounds broke 'whatchanged -p'.  In attempting to fix this,
make two axis of output selection in rev-parse orthogonal:

  --revs-only	tells it not to output things that are not revisions nor
		flags that rev-list would take.
  --no-revs	tells it not to output things that are revisions or
		flags that rev-list would take.
  --flags	tells it not to output parameters that do not start with
		a '-'.
  --no-flags	tells it not to output parameters that starts with a '-'.

So for example 'rev-parse --no-revs -p arch/i386' would yield '-p arch/i386',
while 'rev-parse --no-revs --flags -p archi/i386' would give just '-p'.

Also the meaning of --verify has been made stronger.  It now rejects
anything but a single valid rev argument.  Earlier it passed some flags
through without complaining.

Signed-off-by: Junio C Hamano <junkio@cox.net>