Don't allow combination of -g and --reverse as it doesn't work

Don't allow combination of -g and --reverse as it doesn't work

The --walk-reflogs logic and the --reverse logic are completely
incompatible with one another.  Attempting to use both at the same
time leads to confusing results that sometimes violates the user's
formatting options or ignores the user's request to see the reflog
message and timestamp.

Unfortunately the implementation of both of these features is glued
onto the side of the revision walking machinary in such a way that
they are probably not going to be easy to make them compatible with
each other.  Rather than offering the user confusing results we are
better off bailing out with an error message until such a time as
the implementations can be refactored to be compatible.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>