Clarify output handling in sshrun

Make it clear in the code, that and why we don't expect any captured
output from the SSH subprocess. So, future changes (and debuggers)
don't falsely assume that
1. Output can simply be captured (with existing protocols) or
2. The returned value would currently be of any use simply b/c it's
   there.

datalad/support/sshconnector.py

@@ -174,11 +174,13 @@ def __call__(self, cmd, options=None, stdin=None, log_output=True):
           (probably most) of the available configuration options should not be
           set here because they can critically change the properties of the
           connection. This exists to allow options like SendEnv to be set.
+        log_output: bool
+          Whether to capture and return stdout+stderr.
 
         Returns
         -------
         tuple of str
-          stdout, stderr of the command run.
+          stdout, stderr of the command run, if `log_output` was `True`
         """
         raise NotImplementedError
 

datalad/support/sshrun.py

@@ -116,8 +116,11 @@ def __call__(login, cmd,
         # use an empty temp file as stdin if none shall be connected
         stdin_ = tempfile.TemporaryFile() if no_stdin else sys.stdin
         try:
-            out, err = ssh(cmd, stdin=stdin_, log_output=False,
-                           options=options)
+            # We pipe the SSH process' stdout/stderr by means of
+            # `log_output=False`. That's necessary to let callers - for example
+            # git-clone - communicate with the SSH process. Hence, we expect no
+            # output being returned from this call:
+            ssh(cmd, stdin=stdin_, log_output=False, options=options)
         finally:
             if no_stdin:
                 stdin_.close()