Documentation/i18n: quote double-dash for AsciiDoc

As explained in v1.7.3-rc0~13^2 (Work around em-dash handling in newer
AsciiDoc, 2010-08-23), if double dashes in names of commands are not
escaped, AsciiDoc renders them as em dashes.

While fixing that, spell the command name as "git sh-i18n--envsubst"
(2 words) instead of emphasizing the name of the binary (one
hyphenated name) and format it in italics.

The double-dash in the title should be escaped, too, to avoid spurious
em dashes in the header:

  .TH "GIT\-SH\-I18N\(emENVSUB" "1" "06/26/2011" "Git 1\&.7\&.6" "Git Manual"

AsciiDoc 8.6.4 with DocBook XSL 1.76.0-RC1 copes fine and writes
"GIT\-SH\-I18N\-\-ENVSUB" even without this change, which is why it
was missed before.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-sh-i18n--envsubst.txt

@@ -1,5 +1,5 @@
-git-sh-i18n--envsubst(1)
-========================
+git-sh-i18n{litdd}envsubst(1)
+=============================
 
 NAME
 ----
@@ -10,8 +10,8 @@ SYNOPSIS
 [verse]
 eval_gettext () {
 	printf "%s" "$1" | (
-		export PATH $('git sh-i18n--envsubst' --variables "$1");
-		'git sh-i18n--envsubst' "$1"
+		export PATH $('git sh-i18n{litdd}envsubst' --variables "$1");
+		'git sh-i18n{litdd}envsubst' "$1"
 	)
 }
 
@@ -22,7 +22,7 @@ This is not a command the end user would want to run.  Ever.
 This documentation is meant for people who are studying the
 plumbing scripts and/or are writing new ones.
 
-git-sh-i18n--envsubst is Git's stripped-down copy of the GNU
+'git sh-i18n{litdd}envsubst' is Git's stripped-down copy of the GNU
 `envsubst(1)` program that comes with the GNU gettext package. It's
 used internally by linkgit:git-sh-i18n[1] to interpolate the variables
 passed to the the `eval_gettext` function.

Documentation/git-sh-i18n.txt

@@ -34,7 +34,7 @@ gettext::
 eval_gettext::
 	Currently a dummy fall-through function implemented as a wrapper
 	around `printf(1)` with variables expanded by the
-	linkgit:git-sh-i18n--envsubst[1] helper. Will be replaced by a
+	linkgit:git-sh-i18n{litdd}envsubst[1] helper. Will be replaced by a
 	real gettext implementation in a later version.
 
 GIT