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 git-for-windows#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>

git-gui--askpass

@@ -56,6 +56,11 @@ proc finish {} {
 		}
 	}
 
+	# On Windows, force the encoding to UTF-8: it is what `git.exe` expects
+	if {$::tcl_platform(platform) eq {windows}} {
+		set ::answer [encoding convertto utf-8 $::answer]
+	}
+
 	puts $::answer
 	set ::rc 0
 }