hgweb: use an extensible list of files to check for refresh
The refresh feature was explicitly testing if '00changelog.i' and 'phaseroots'
changed. This is overlooking other important information like bookmarks and
obsstore (bookmark have their own hack to work around it).
We move to a more extensible system with a list of files of interest
that will be used to build the repo state. The system should probably
move into a more central place so that the command server and other
systems are able to use it. Extension writers will also be able to add
entries to ensure that changes to extension data are properly detected.
Also the current key (mtime, size) is notably weak for bookmarks and phases
whose files can easily change content without effect on their size.
Still, this patch seems like a valuable minimal first step.