fix: avoid eval "= directly in builtin registers picker (#2347)

When "= contains expression not ought to be run in current context, the
`getreg('=')` raises error.

For example `:put =foo` (where `foo` is undefined) then open registers
picker it fails. Also #2228.

Now specify second arg to `getreg` to get register expr (instead of
"evaluated" result) as content, like what `:registers` command displays.

This has behavior change, if user still want to see/paste the
evaluated result, a customized action will be needed.

lua/telescope/make_entry.lua

@@ -820,7 +820,7 @@ function make_entry.gen_from_registers(opts)
   end
 
   return function(entry)
-    local contents = vim.fn.getreg(entry)
+    local contents = vim.fn.getreg(entry, 1)
     return make_entry.set_default_entry_mt({
       value = entry,
       ordinal = string.format("%s %s", entry, contents),