hgweb: refresh hgweb.repo on phase change (issue4061)
Make hgweb.refresh() also look at phaseroots file (in addition to 00changelog.i
file) and reload the repo when os.stat returns different mtime or size than
cached, signifying the file was modified.
This way if user changes phase of a changeset (secret <-> draft), there's no
need to restart hg serve to see the change.