Cloning with protected keys

Similar to #94 , rather than AuthFailed I'm getting complaints about "session down" when my key is protected. Standard git clone of the same repo works fine and triggers password prompt.

```
Caused by: org.eclipse.jgit.errors.TransportException: git@github.com:pmandera/basic-scala-project.g8.git: session is down
at org.eclipse.jgit.transport.JschSession$JschProcess.(JschSession.java:154)
at org.eclipse.jgit.transport.JschSession$JschProcess.(JschSession.java:118)
at org.eclipse.jgit.transport.JschSession.exec(JschSession.java:91)
at org.eclipse.jgit.transport.TransportGitSsh$SshFetchConnection.(TransportGitSsh.java:248)
at org.eclipse.jgit.transport.TransportGitSsh.openFetch(TransportGitSsh.java:147)
at org.eclipse.jgit.transport.FetchProcess.executeImp(FetchProcess.java:127)
at org.eclipse.jgit.transport.FetchProcess.execute(FetchProcess.java:113)
at org.eclipse.jgit.transport.Transport.fetch(Transport.java:1062)
at org.eclipse.jgit.api.FetchCommand.call(FetchCommand.java:129)
... 28 more
Caused by: com.jcraft.jsch.JSchException: session is down
at com.jcraft.jsch.Session.openChannel(Session.java:762)
at org.eclipse.jgit.transport.JschSession$JschProcess.(JschSession.java:147)
... 36 more
Error during sbt execution: org.eclipse.jgit.api.errors.JGitInternalException: Exception caught during execution of fetch command
```

Start with issue #94 and the JGit/JSch stack trace, then compare giter8's SSH clone path with standard git when a protected key is used. Done means cloning the reported repository succeeds and prompts for the key password instead of reporting that the session is down.

Written by the indexing model from the issue text.