git: use /bin/true for 'askpass' to prevent all password prompts

git: use /bin/true for 'askpass' to prevent all password prompts

This uses both the env vars and the command line options to ensure
that it works with as many versions of git as possible.  Also, git-svn
uses the env vars, but not necessarily the command line options.

This uses /bin/true to pretend that it succesfully got the password.
If password auth is truly required, then it will fail further on down
the line.