git-gui--askpass: coerce answers to UTF-8 on Windows

git-gui--askpass: coerce answers to UTF-8 on Windows

This addresses the issue where Git for Windows asks the user for a
password, no credential helper is available, and then Git fails to pick
up non-ASCII characters from the Git GUI helper.

This can be verified e.g. via

	echo host=http://abc.com |
	git -c credential.helper= credential fill

and then pasting some umlauts.

The underlying reason is that Git for Windows tries to communicate using
the UTF-8 encoding no matter what the actual current code page is. So
let's indulge Git for Windows and do use that encoding.

This fixes https://github.com/git-for-windows/git/issues/2215

Signed-off-by: Luke Bonanomi <lbonanomi@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>