setup_revisions() wants to get all the parameters at once and
then postprocesses the resulting revs structure after it is done
with them.  This code structure is a bit cumbersome to deal with
efficiently when we want to inject revision parameters from the
side (e.g. read from standard input).

Fortunately, the nature of this postprocessing is not affected by
revision parameters; they are affected only by flags.  So it is
Ok to do add_object() after the it returns.

This splits out the code that deals with the revision parameter
out of the main loop of setup_revisions(), so that we can later
call it from elsewhere after it returns.
Signed-off-by: NJunio C Hamano <junkio@cox.net>