vim-patch:8.1.0956: context:0 in 'diffopt' #10622

Problem:    Using context:0 in 'diffopt' does not work well.
Solution:   Make zero context do the same as one line context. (closes vim/vim#4005)
b9ddda6c2d

src/nvim/diff.c

@@ -2190,7 +2190,7 @@ int diffopt_changed(void)
   }
 
   diff_flags = diff_flags_new;
-  diff_context = diff_context_new;
+  diff_context = diff_context_new == 0 ? 1 : diff_context_new;
   diff_foldcolumn = diff_foldcolumn_new;
   diff_algorithm = diff_algorithm_new;