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