Add anchor "mw-oldid" for beginning of page content in diff view

Add anchor "mw-oldid" for beginning of page content in diff view

* #top, #mw-content-text and others exist, but all point to the
  beginning of the diff, not of the actual page content.
* Pick mw- prefix and technical but familiar related name, in
  the hope it's not confusing nor too hard to remember.
* Lowercase letters, similar to #top, #footer, #file etc. were also
  considered and discard, inspired to usual names:
  https://www.wikidata.org/wiki/Q10966628

Bug: T4313
Change-Id: Ia90f2541327480535e9943ac30bfa19791a19620