Reduce code duplication correctly this time, again

Reduce code duplication correctly this time, again

The test cases I thought up are at:

http://www.mediawiki.org/wiki/User:Simetrical/Id_tests

All of them pass with the patch, except for some that fail on current
code as well: the ones involving templates, multiply-occurring section
headers, or numeric id's (there seems to be a weird bug with those that
probably involves string and numeric id's being used in the same array).
This is true whether $wgEnforceHtmlIds is on or off.  (Actually, the
problem with numeric keys doesn't happen with $wgEnforceHtmlIds off,
because of course numeric ids aren't allowed then.)