builtin-grep: do not use setup_revisions()

builtin-grep: do not use setup_revisions()

Grep may want to grok multiple revisions, but it does not make
much sense to walk revisions while doing so.  This stops calling
the code to parse parameters for the revision walker.  The
parameter parsing for the optional "-e" option becomes a lot
simpler with it as well.

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