This is a schema change. It's only a table creation, but the table must be created...

This is a schema change.  It's only a table creation, but the table must be created on Wikimedia servers before this revision goes live.  The maintenance script populateCategory.php should be run when convenient.  If it's not run, there's only one substantial case where display will be harmed: the page of a category with more than 200 net pages added since the patch goes live will give an erroneously low count.  In other cases category pages will just be better-worded, and it will recognize the count in the table is bogus.
* Adds Category and CategoryList classes to represent categories themselves.
* Adds a category table, giving each category a name, ID, and counts of all members, subcats only, and files.
* Adds a maintenance script to populate the category table efficiently.  This script is careful to wait for slaves and should be safe to run on a live database.  The maintenance script's includes file is called by update.php.
* Until the category table is populated, the patch handles weird category table rows gracefully.  It detects whether they're obviously impossible, and if so, it outputs appropriate messages.