i18n: show the non-ASCII password prompt text correctly

Before this patch, the prompt text for asking password is directly
passed to "getpass.getpass()" of Python standard library.

In "getpass.getpass()" implementation on Windows environment, the
prompt text is split into byte sequence and "msvcrt.putch()" is
applied on each bytes in it. This splitting causes non-ASCII prompt
text to be broken.

This patch shows the prompt text for asking password on "ui.getpass()"
side, and invokes "getpass.getpass()" with empty prompt text. This
prevents non-ASCII prompt text from being broken in
"getpass.getpass()" implementation.

This patch also sets "ui.prompt" label to prompt text to follow
"ui.prompt()" style.

--HG--
branch : stable