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.
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors