parser: reorder alias expansion routine to return early I think it improves readability to move trivial cases first, and unindent blocks.

parser: move functions that process alias expansion to rule-set class They will be commonly used by revset and templater. It isn't easy to understand how _expand() works, so I'll add comments by a follow-up patch. The local variable 'alias' is renamed to 'a' to avoid shadowing the global 'alias' class.

revset: unindent codes in _getalias() function We generally do return early if tree isn't a tuple.