Add more aliases for Special:Diff for language 'de'
The local community wants 'Diff' as canonical form, add also the long
form for what Diff stands.
Keep the old alias, was added with:
Ic2b34a948d527cee6b76abd7e24a6d8a33b65bd2
https://translatewiki.net/w/i.php?title=MediaWiki:Sp-translate-data-SpecialPageAliases/de&diff=
6066852&oldid=
6049012
https://de.wikipedia.org/w/index.php?title=Wikipedia:Fragen_zur_Wikipedia&oldid=
139947048#Spezial:Diff_.E2.86.92_Spezial:Unterschied
Change-Id: I504cd4b0e7c7b5c22812fe9eb0cf8dec9ff9514b