vim-patch:9.1.1600: using diff anchors with hidden buffers fails silently (#35218)

Problem:  diff: using diff anchors with hidden buffers fails silently
Solution: Give specific error message for diff anchors when using hidden
          buffers (Yee Cheng Chin).

Diff anchors currently will fail to parse if a buffer used for diff'ing
is hidden. Previously it would just fail as the code assumes it would
not happen normally, but this is actually possible to do if `closeoff`
and `hideoff` are not set in diffopt. Git's default diff tool "vimdiff3"
also takes advantage of this.

This fix this properly would require the `{address}` parser to be
smarter about whether a particular address relies on window position or
not (e.g. the `'.` address requires an active window, but `'a` or `1234`
do not). Since hidden diff buffers seem relatively niche, just provide a
better error message / documentation for now. This could be improved
later if there's a demand for it.

related: vim/vim#17615
closes: vim/vim#17904

cad3b2421d

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

src/nvim/options.lua

@@ -2286,6 +2286,8 @@ local options = {
         If some of the {address} do not resolve to a line in each buffer (e.g.
         a pattern search that does not match anything), none of the anchors
         will be used.
+        							*E1562*
+        Diff anchors can only be used when there are no hidden diff buffers.
       ]=],
       full_name = 'diffanchors',
       list = 'onecomma',