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>

runtime/lua/vim/_meta/options.lua

@@ -1736,6 +1736,8 @@ vim.wo.diff = vim.o.diff
 --- 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.
 ---
 --- @type string
 vim.o.diffanchors = ""