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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).