When GIT_SSH is set to use plink, the correct option name is "-batch"
and not "--batch". This was a typo introduced when we added support
for plink via GIT_SSH.
Change-Id: I391660e38f5d208bba11e3f2a8f25922de2af878
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>