git-svn: Support retrieving passwords with GIT_ASKPASS

git-svn: Support retrieving passwords with GIT_ASKPASS

git-svn reads passwords from an interactive terminal.
This behavior cause GUIs to hang waiting for git-svn to
complete

Fix this problem by allowing a password-retrieving command
to be specified in GIT_ASKPASS. SSH_ASKPASS is supported
as a fallback when GIT_ASKPASS is not provided.

Signed-off-by: Frank Li <lznuaa@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>