R1:f56f1b05c0dd

Install a SIGTERM handler in ssh-connect

Summary:
Ref T10547. This has been around for a while but I was never able to reproduce it. I caught a repro case in the cluster recently and I think this is the right fix.

We tell Subversion to run `ssh-connect` instead of `ssh` so we can provide options and credentials, by using `SVN_SSH` in the environment. Subversion will sometimes kill the SSH tunnel subprocess aggressively with SIGTERM -- as of writing, you can search for `SIGTERM` in `make_tunnel()` here…