Assign tagid to SelfclosingTagTk as well to detect fostered cat-links

* Category links are represented as SelfclosingTagTk and they can
  get fostered ==> they need a tag-id so that their fostering
  can be detected by the algo in git sha 89057946294cc.

* Currently, category links are being fostered out a lot on pages
  like en:List_of_X-Men_(TV_series)_episodes (a subset of pages that
  use Template:Episode_list).  However, the fix in 89057946294cc wasn't
  detecting this.  The fix in this patch ensures that those fostered
  links are ignored properly by selser and don't cause dirty diffs
  as seen in:
  https://en.wikipedia.org/w/index.php?title=List_of_X-Men_%28TV_series%29_episodes&diff=565487327&oldid=565419567

* No change in parser tests.

* Verified fix by testing on a use of Template:Episode_list
  in a table, introducing edit changes, and verifying that the
  fostered links are ignored.

* To be separately investigated: the fostered category link comes
  from a template and is currently escaping template encapsulation.

Change-Id: I4e998ebd38d3fc176414c9083d42dbb4b7c31e16