transplant: use "ui.promptchoice()" for interactive transplant Before this patch, transplant extension uses "ui.prompt()" for interactive transplant, and has to check whether user response returned by "ui.prompt()" is valid or not in own code. In addition to it, transplant extension uses response characters (e.g. "y", "n", and so on) directly in own code, and this disallows to use another response characters by translation, even though the help shown by '?' typing is translatable. This patch uses "ui.promptchoice()" instead of "ui.prompt()" to resolve problems above.