Replace the "This manual page describes only the most frequently used options."
text with the list of rev-list options in git-log manpage. (The git-diff-tree
options are already included.)

Move these options to a separate file and include it from both
git-rev-list.txt and git-log.txt.
Signed-off-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>