asciidoc markup fixes

asciidoc markup fixes

I see quite a few pages on k.org site, e.g.

    http://www.kernel.org/pub/software/scm/git/docs/git-rerere.html
    (scroll down to find "After this test merge")

are misformatted to lose teletype text '+' that is followed by a comma,
and turns the following paragraph all typeset in teletype.

This patch seems to fix the issue at the site (meaning, with the
particular vintage of asciidoc and docbook toolchain), without breaking
things with the version I have at my primary development machine, but
wider testing is very much appreciated.

After this patch,

    git grep '`+`,' -- Documentation

should report noting.

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