Restore registers with setreg()

Do not use "let @a = _a" to restore registers, since this adds an extra line
feed character to the register.

References:
- http://stackoverflow.com/questions/26896399/why-does-let-modify-the-value-it-assigns
- https://bugzilla.redhat.com/show_bug.cgi?id=1163518

Patch idea by Daniel Wagner <me@dmwit.com>