hgweb: move common vertex code to Graph.prototype
Just to give some context to the return values: vertex() needs to return two
HTML elements as strings, <li> to be used as a background and a <li> to be
shown in foreground. The latter was made obsolete recently when changesets
started to be rendered server-side, but background elements are still useful
for now.
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif