Implement the 'goto' command

Push/pop patches to/from the stack until the one given on the command
line becomes current. This is a shortcut for the 'push --to' or 'pop
--to' commands. There is no '--undo' option for 'goto'. Use the 'push'
command for this.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>