filter/rubylang: reuse altid entry from inbox object

filter/rubylang: reuse altid entry from inbox object

This allows users to DRY up their config a bit and avoid
specifying altid twice when reusing the NNTP-centric msgmap
for [ruby-*:\d+] serial numbers.

My current work-in-progress ~/.public-inbox/config entry
for the ruby-core list is:

------8<-------
[publicinbox "ruby-core"]
	address = ruby-core@ruby-lang.org
	url = //public-inbox.org/ruby-core
	mainrepo = /path/to/ruby-core.git
	newsgroup = inbox.comp.lang.ruby.core
	watchheader = List-Id:<ruby-core.ruby-lang.org>
	altid = serial:ruby-core:file=msgmap.sqlite3
	watch = maildir:/path/to/Maildir/.INBOX.ruby
	filter = PublicInbox::Filter::RubyLang