remote-testgit: properly check for errors

remote-testgit: properly check for errors

'feature done' was missing, which allowed fast-import exit properly, and
transport-helper to continue checking for refs and what not when in fact
the remote-helper died.

Let's enable that, and make sure the error paths are triggered.

Now transport-helper correctly detects the errors from fast-import,
unfortunately, not from fast-export because it might finish before
detecting a SIGPIPE. This means transport-helper will quit silently and
the user will not see any errors, which is bad. Hopefully the helper
will print the error before dying anyway, so not all is lost.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>