portability fix for test-ssh
The __del__ method of sshrepo reads the stderr of the remote process
until EOF and prints it.
If an exception is raised, this method ends up being called:
- on Linux: after the "abort: ..." message is printed
- on OS X: before the "abort: ..." message is printed
This patch explicitly flushes the stderr of the remote process before
raising a RepoError.
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