glossary: fix overoptimistic automatic linking of defined terms

glossary: fix overoptimistic automatic linking of defined terms

The script sort_glossary.pl turns each use of "term" into a link to the
definition of "term".  To avoid mangling links like

	gitlink:git-term[1]

it doesn't replace any occurence of "term" preceded by "link:git-".
This fails for gitlink:git-symbolic-ref[1] when substituting for "ref".

So instead just refuse to replace anything preceded by a "-".
That could result in missing some opportunities, but that's a less
annoying error.

Actually I find the automatic substitution a little distracting; some
day maybe we should just run it once and commit the result, so it can
be hand-tuned.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>