Documentation: quote double-dash for AsciiDoc

Documentation: quote double-dash for AsciiDoc

AsciiDoc versions since 5.0.6 treat a double-dash surrounded by spaces
(outside of verbatim environments) as a request to insert an em dash.
Such versions also treat the three-character sequence "\--", when not
followed by another dash, as a request to insert two literal minus
signs.  Thus from time to time there have been patches to add
backslashes to AsciiDoc markup to escape double-dashes that are meant
to be represent '--' characters used literally on the command line;
see v1.4.0-rc1~174, Fix up docs where "--" isn't displayed correctly,
2006-05-05, for example.

AsciiDoc 6.0.3 (2005-04-20) made life harder by also treating
double-dashes without surrounding whitespace as markup for an em dash,
though only when formatting for backends other than the manpages
(e.g., HTML).  Many pages needed to be changed to use a backslash
before the "--" in names of command-line flags like "--add" (see
v0.99.6~37, Update tutorial, 2005-08-30).

AsciiDoc 8.3.0 (2008-11-29) refined the em-dash rule to avoid that
requirement.  Double-dashes without surrounding spaces are not
rendered as em dashes any more unless bordered on both sides by
alphanumeric characters.  The unescaped markup for option names (e.g.,
"--add") works fine, and many instances of this style have leaked into
Documentation/; git's HTML documentation contains many spurious em
dashes when formatted by an older toolchain.  (This patch will not
change that.)

The upshot: "--" as an isolated word and in phrases like "git
web--browse" must be escaped if it is not to be rendered as an em dash
by current asciidoc.  Use "\--" to avoid such misformatting in
sentences in which "--" represents a literal double-minus command line
argument that separates options and revs from pathspecs, and use
"{litdd}" in cases where the double-dash is embedded in the command
name.  The latter is just for consistency with v1.7.3-rc0~13^2 (Work
around em-dash handling in newer AsciiDoc, 2010-08-23).

List of lines to fix found by grepping manpages for "(em".

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Improved-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>