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>