Extract "log [diff options]" parser out.

Extract "log [diff options]" parser out.

Merging of the log-tree-opt structure with rev-info structure
did not work out very well and it broke things that did not want
diff options and/or rev options.

This is an alternative approach to define a combined interface
that can be used by commands that wants both.  The use of it is
opt-in to reduce the risk of breaking existing programs.

We might want to slurp "setup_revisions() places things in
pending objects list" part from Linus's earlier attempt.

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