Allow passing extra set of arguments when ssh is invoked to create
an encrypted & authenticated connection by introducing a new environment
variable GIT_SSH_COMMAND, whose contents is interpreted by shells.
This is not possible with existing GIT_SSH mechanism whose
invocation bypasses shells, which was designed more to match what
other programs with similar variables did, not necessarily to be
more useful.
* tq/git-ssh-command:
git_connect: set ssh shell command in GIT_SSH_COMMAND