connect.c: Support PuTTY plink and TortoisePlink as SSH on Windows

connect.c: Support PuTTY plink and TortoisePlink as SSH on Windows

OpenSSH uses -p to specify custom ports, while PuTTY plink and
TortoisePlink use -P. Git now detects if plink is in GIT_SSH and
modify its flags as necessary.

We call plink with -batch, so that it will error out with an error
message instead of waiting for user input.  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, since
plink tries to get the answer from stdin.  However, stdin is
already connected to Git that wants to send input to the remote
command.

But we do not pass -batch to TortoisePlink, because TortoisePlink
uses a GUI to communicate with the user, and it does not understand
-batch.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>