Error out rather than hang when plink does not know the host yet
As reported in msysGit issue 96, plink wants to interact with the user
asking if a host key should be accepted, but this just blocks the
terminal, as plink tries to get the answer from stdin. However, stdin
is already connected to Git that wants to send input to the remote
command.
Rather than blocking, just call plink with -batch so that it will error
out with an error message.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>