git_connect: set ssh shell command in GIT_SSH_COMMAND

git_connect: set ssh shell command in GIT_SSH_COMMAND

It may be impractical to install a wrapper script for GIT_SSH
when additional parameters need to be passed. Provide an alternative
way of specifying a shell command to be run, including command line
arguments, by means of the GIT_SSH_COMMAND environment variable,
which behaves like GIT_SSH but is passed to the shell.

The special circuitry to modify parameters in the case of using
PuTTY's plink/tortoiseplink is activated only when using GIT_SSH;
in the case of using GIT_SSH_COMMAND, it is deliberately left up to
the user to make any required parameters adaptation before calling
the underlying ssh implementation.

Signed-off-by: Thomas Quinot <thomas@quinot.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>