Documentation doesn't build anymore on old asciidoc due to i18n series

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

I can submit a patch for this later, just thought I'd note it and
solicit feedback on the best solution to this.

On my Debian box with asciidoc 8.6.4 Git builds just fine from master.

But on a Centos 5.5 box with asciidoc 8.2.5 I get:

    rm -f git-sh-i18n--envsubst.html+ git-sh-i18n--envsubst.html && \
            asciidoc -b xhtml11 -d manpage -f asciidoc.conf \
                     -a asciidoc7compatible -a no-inline-literal -a
git-asciidoc-no-roff -agit_version=1.7.5.3.367.ga9930 -o
git-sh-i18n--envsubst.html+ git-sh-i18n--envsubst.txt && \
            mv git-sh-i18n--envsubst.html+ git-sh-i18n--envsubst.html
    ERROR: git-sh-i18n--envsubst.txt: line 9: second section must be
named SYNOPSIS
    make[1]: *** [git-sh-i18n--envsubst.html] Error 1

The solution is obvious: just add a SYNOPSIS section for
consistency. I'll do that later, unless we want to provide some option
(which old asciidoc may or may not support) to not have a SYNOPSIS
there, which we arguably can just omit for purely internal tooling
like this.

I'll submit patches soon for adding a SYNOPSIS unless we want to go
that route.