runtime(doc): disable last-position-jump in diff mode

This has been bothering me quite for some time and I never knew why it
happened. Just today it occurred to me this might have been because of
the last-position-jump.

So I figured, let's fix it for everybody, not just me.

closes: #17092

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/defaults.vim

@@ -107,6 +107,7 @@ if 1
       \ let line = line("'\"")
       \ | if line >= 1 && line <= line("$") && &filetype !~# 'commit'
       \      && index(['xxd', 'gitrebase', 'tutor'], &filetype) == -1
+      \      && !&diff
       \ |   execute "normal! g`\""
       \ | endif