revset: introduce _parsealiasdefn to parse alias definitions strictly This patch introduces "_parsealiasdefn" to parse alias definitions strictly. For example, it can avoid problems below, which current implementation can't. - the shorter name argument breaks referring the longer name one in the definition, if the former is completely prefix of the latter for example, the alias definition "foo($1, $10) = $1 or $10" is parsed as "_aliasarg('$1') or _aliasarg('$1')0" and causes parse error, because tail "0" of "_aliasarg('$1')0" is invalid. - argument names in the quoted string are broken for example, the definition "foo($1) = $1 or desc('$1')" is parsed as "_aliasarg('$1') or desc('_aliasarg(\'$1\')')" and causes unexpected description matching against not '$1' but '_aliasarg(\'$1\')'. To decrease complication of patch, current implementation for alias definitions is replaced by "_parsealiasdefn" in the subsequent patch. This patch just introduces it. This patch defines "_parsealiasdefn" not as a method of "revsetalias" class but as a one of "revset" module, because of ease of testing by doctest.