git: use a custom script for GIT_ASKPASS instead of true

git: use a custom script for GIT_ASKPASS instead of true

Setting GIT_ASKPASS to true should result in an empty password.

However, since it doesn't actually output anything, there's no
trailing \n after the empty password.

This causes problems for git-svn that expects the password to
be followed by a \n and can cause unsightly core dumps therein.

Instead set GIT_ASKPASS to a custom script that outputs the
value of the GIT_ASKPASS_PASSWORD environment variable followed
by a newline.  This makes git-svn happy and allows a non-empty
password to be easily passed from a script if desired.