MinGW: Add a simple getpass()

MinGW: Add a simple getpass()

We need getpass() to activate curl on MinGW.  Although the default
Makefile currently has 'NO_CURL = YesPlease', msysgit releases do
provide curl support, so getpass() is used.

[spr: - edited commit message.
      - squashed commit that provides getpass() declaration.]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>