gitglossary: fix indentation of sub-lists

gitglossary: fix indentation of sub-lists

The glossary entry is a list of terms and their definitions, so
multi-paragraph definitions need "+" continuation lines to indicate
that they are part of a single entry.

When an entry contains a sub-list (say, a bulleted list), the final "+"
may become ambiguous: is it connecting the next paragraph to the final
entry of the sub-list, or to the original list of definition paragraphs?

Asciidoc generally connects it to the former, even when we mean the
latter, and you end up with the next paragraph indented incorrectly,
like this:

  glob
    ...defines glob...

    Two consecutive asterisks ("**") in patterns matched
    against full pathname may have special meaning:

    - ...some special meaning of **...

    - ...another special meaning of **...

    - Other consecutive asterisks are considered invalid.

      Glob magic is incompatible with literal magic.

That final "Glob magic is incompatible" paragraph is in the wrong spot.
It should be at the same level as "Two consecutive asterisks", as it is
not part of the final "Other consecutive asterisks" bullet point.

The same problem appears in several other spots in the glossary.

Usually we'd fix this by using "--" markers, which put the sub-list into
its own block. But there's a catch: in some of these spots we are
already in an open block, and nesting open blocks is a problem. It seems
to work for me using Asciidoc 10.2.1, but Asciidoctor 2.0.26 makes a
mess of it (our intent to open a new block seems to close the old one).

Fortunately there's a work-around: when using a "+" list-continuation,
the number of empty lines above the continuation indicates which level
of parent list to continue. So by adding an empty line after our
unordered list (before the "+"), we should be able to continue the
definition list item.

But asciidoc being asciidoc, of course that is not the end of the story.
That technique works fine for the "glob" and "attr" lists in this patch,
but under the "refs" item it works for only 1 of the 2 lists! I can't
figure out why, and this may be an asciidoctor bug. But we can work
around it by using "--" open-block markers here, since we're not
already in an open block.

So using the extra blank line for the first two instances, and "--"
markers for the second two, this patch produces identical output from
"doc-diff HEAD^ HEAD" for both --asciidoctor and --asciidoc modes.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>