hgweb: add revset syntax support to search This mode is used when all the conditions are met: - 'reverse(%s)' % query string can be parsed to a revset tree - this tree has depth more than two, i.e. the query has some part of revset syntax used - the repo can be actually matched against this tree, i.e. it has only existent function/operators and revisions/tags/bookmarks specified are correct - no revset regexes are used in the query (strings which start with 're:') - only functions explicitly marked as safe in revset.py are used in the query Add several new tests for different parsing conditions and exception handling.