hgweb: optimize process_dates function

This function looped over every node due to getElementsByTagName('*'), instead
of using selectors. In this patch we use querySelectorAll('.age') and process
only these nodes, which is much faster and also doesn't require extra condition.

Browser compatibility isn't sacrificed: IE 8+, FF 3.5+, Opera 10+.
hgweb: add parentSelector argument to process_dates

Allow specifying parent selector of elements to process, useful for
incremental page updates.
hgweb: call process_dates with a specified selector in ajax scroll

Now this function processes only newly added entries, and not old ones,
the amount of which can be much bigger.