Work around em-dash handling in newer AsciiDoc

Work around em-dash handling in newer AsciiDoc

Older versions of AsciiDoc used to literally pass double dashes when we
used them in our linkgit macros and manpage titles, but newer ones (the
issue was first reported with AsciiDoc 8.5.2) turn them into em dashes.

Define litdd (literal double-dash) custom attribute in asciidoc.conf to
work this around.  While we are at it, fix a few double-dashes (e.g. the
description of "project--devo--version" convention used by tla, among
other things) that used to be incorrectly written as em dashes in the body
text to also use this attribute.

Signed-off-by: Junio C Hamano <gitster@pobox.com>