make configuration via vim global variables more consistent

options in vim global variables no longer override:

- nested keys in `default_opts`
- `setup` `opts` with default-equivalent values

lua/gitblame/config.lua

@@ -27,13 +27,16 @@ M.default_opts = {
 
 ---@param opts SetupOptions?
 M.setup = function(opts)
-    opts = opts or {}
-    opts = vim.tbl_deep_extend("force", M.default_opts, opts)
+    local opts = opts or {}
 
-    for key, value in pairs(opts) do
-        if vim.g["gitblame_" .. key] == nil or M.default_opts[key] ~= value then
-            vim.g["gitblame_" .. key] = value
-        end
+    local global_var_opts = {}
+    for k, _ in pairs(M.default_opts) do
+        global_var_opts[k] = vim.g["gitblame_" .. k]
+    end
+
+    opts = vim.tbl_deep_extend("force", M.default_opts, global_var_opts, opts)
+    for k, v in pairs(opts) do
+        vim.g["gitblame_" .. k] = v
     end
 end