revset: flatten chained 'list' operations (aka function args) (issue5072)
Internal _matchfiles() function can take bunch of arguments, which would
lead to a maximum recursion depth error. This patch avoids the excessive
stack use by flattening 'list' nodes beforehand.
Since getlist() no longer takes a nested 'list' nodes, _parsealiasdecl()
also needs to flatten argument list, "aliasname($1, $2, ...)".