hgweb: add a link to followlines in descending direction
We change the content of the followlines popup to display two links inviting
to follow the history of selected lines in ascending (as before) and
descending directions. The popup now renders as:
follow history of lines <fromline>:<toline>:
<a href=...>ascending</a> / <a href=...>descending</a>
#require docutils gettext
$ $TESTDIR/check-gendoc zh_TW
checking for parse errors