Set GIT_ASKPASS=true when running Git

Set GIT_ASKPASS=true when running Git

Previously we set GIT_TERMINAL_PROMPT=0 to cause any fetches that
result in remote 401 errors to fail immediately rather than requesting
a password.  That only works with newer Gits.  To effect the same
result with older Gits we set GIT_ASKPASS to true assuming that
executable is somewhere in the PATH and ignores any arguments and
always returns success.  This causes an empty password to be sent
rather than immediately aborting but it's probably almost as
effective in most cases.