doc/git-log: move "Diff Formatting" from rev-list-options

doc/git-log: move "Diff Formatting" from rev-list-options

Our rev-list-options.txt include has a "Diff Formatting" section, but it
is ifndef'd out for all manpages except git-log. And a few bits of the
text are rather out of date.

We say "some of these options are specific to git-rev-list". That's
obviously silly since we (even before this patch) show the content only
for git-log. But moreover, it's not true; each of the listed options is
meaningful for other diff commands.

We also say "...however other diff options may be given. See git-diff-files
for more options." But there's no need to do so; git-log already has a
"Common Diff Options" section which includes diff-options.txt.

So let's move these options over to git-log and put them with the other
diff options, giving a single "diff" section for the git-log
documentation. We'll call it "Diff Formatting" but use the all-caps
top-level header to match its sibling sections. And we'll rewrite the
section intro to remove the useless bits and give a more generic
overview of the section which can be later extended.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>