Merge branch 'jk/connect-clear-env'

The ssh transport, just like any other transport over the network,
did not clear GIT_* environment variables, but it is possible to
use SendEnv and AcceptEnv to leak them to the remote invocation of
Git, which is not a good idea at all.  Explicitly clear them just
like we do for the local transport.

* jk/connect-clear-env:
  git_connect: clarify conn->use_shell flag
  git_connect: clear GIT_* environment for ssh