Documentation/i18n: quote double-dash for AsciiDoc

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>