revset: explicitely tag alias arguments for expansion
The current revset alias expansion code works like:
1- Get the replacement tree
2- Substitute the variables in the replacement tree
3- Expand the replacement tree
It makes it easy to substitute alias arguments because the placeholders
are always replaced before the updated replacement tree is expanded
again. Unfortunately, to fix other alias expansion issues, we need to
reorder the sequence and delay the argument substitution. To solve this,
a new "virtual" construct called _aliasarg() is introduced and injected
when parsing the aliases definitions. Only _aliasarg() will be
substituted in the argument expansion phase instead of all regular
matching string. We also check user inputs do not contain unexpected
_aliasarg() instances to avoid argument injections.