http getpass function in msysgit

[Frank Li <lznuaa@gmail.com>]

ALL:

        getpass at mingw.c is direct read character with function _getch().
        GUI application, such tortoisegit will halt when http need
password input because gui don't know git wait for inputing passwords.

        To resolve this problem, I have two options.
        Options 1:
                Check if terminal is exist, if exist, using old
method. Otherwise launch internal password dialog.

        Options 2:
                like openSSH,  Check if terminal is not exist and
environment HTTP_ASKPASS exist, if true, run application which
HTTP_ASKPASS point, otherwise using old ways.

       Which one do you prefer,  I can implement it.

best regards
Frank Li