docs: drop asciidoc7compatible flag

docs: drop asciidoc7compatible flag

When we made the switch to supporting asciidoc 8 in 4c7100a
(Documentation: adjust to AsciiDoc 8, 2007-06-14), we were
able to leave most of the documentation intact by defining
asciidoc7compatible.

Since commit 6cf378f (docs: stop using asciidoc no-inline-literal,
2012-04-26), we don't support versions of asciidoc older
than 8.4.1, which is when inline literals were introduced.
Therefore there is not much point in keeping our
documentation compatible with asciidoc 7.

So we are now free to drop the asciidoc7compatible flag and
update the documentation itself to assume asciidoc8.
Fortunately, doing the latter is very easy; we weren't using
any of the constructs impacted by asciidoc7compatible, so
there are no changes to make.

The reason is somewhat subtle. The asciidoc7compatible
affects only super/sub-scripts ("^" and "~") and index
terms. We don't use the latter at all. Nor we do we use the
former, but we did have to protect them from accidental
expansion in constructs like "rev^1". However, all of our
uses of "~" and "^" are either in code blocks (which are
rendered literally), or inside backticks. Prior to 6cf378f,
backticks were not inline literals, and needed proper
quoting. But post-6cf378f, we don't have to worry whether we
are using the old or new rules, as those characters are not
interpreted at all in either case.

I verified that the result of "make install-html
install-man" is identical before and after this patch on
asciidoc 8.6.7.

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