Merge branch 'jk/connect-clear-env' into maint

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