patch 9.1.1600: using diff anchors with hidden buffers fails silently

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: #17615
closes: #17904

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_diffmode.vim

@@ -3483,6 +3483,10 @@ func Test_diffanchors_invalid()
   diffupdate
   call assert_equal('orig_search_pat', @/) " also check we don't pollute the search register
   set diffanchors=1/does_not_exist/
+  call assert_fails('diffupdate', 'E1550:')
+  call assert_equal('orig_search_pat', @/)
+
+  " Hidden buffers are not supported right now
   hide
   call assert_fails('diffupdate', 'E1562:')