This fixes the issue identified in
https://github.com/git-for-windows/git/issues/1498
where Git would not fall back to reading credentials from a Win32
Console when the credentials could not be read from the terminal via the
Bash hack (that is necessary to support running in a MinTTY).
Tested in a Powershell window.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>