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.
b0395c2882