transport/file: fix race condition on test (#267)

Sometimes, the `TestClone` and `TestPush` tests of `transport/file` fail
in travis.

This is due to a race condition caused by an incorrect usage of
`Cmd.Wait()` while reading from the output and error pipes of the
command.

This patch fixes the problem by using `Cmd.CombinedOutput()` instead of
calling `Cmd.Start()` and `Cmd.Wait()` while reading from the output and
error pipes.

Details:

From the `exec` package documentation:

```
Wait will close the pipe after seeing the command exit, so most callers
need not close the pipe themselves; however, an implication is that it
is incorrect to call Wait before all reads from the pipe have completed.
For the same reason, it is incorrect to call Run when using StdoutPipe.
```

In our tests, the old `execAndGetOutput` function was creating two
gorutines to read from the stderr and stdout pipes of the command and
then call `Wait` on the command.

This caused a race condition: when the `Wait` call finished before the
gorutines have read from the pipes, they returned caused an error on
`outErr`.

The problem only happens sometimes on travis.

To reproduce the problem locally, just add a call to
time.Sleep(time.Second) to the gorutine before its `ioutil.ReadAll` call
to delay them, then `Wait` will always finish before them, closing the
pipes, and the gorutines will fail.  The returned error detected by the
test will be:

```
FAIL: server_test.go:55: ServerSuite.TestClone
server_test.go:65:
    c.Assert(err, IsNil, Commentf("STDOUT:\n%s\nSTDERR:\n%s\n", stdout, stderr))
    ... value *os.PathError = &os.PathError{Op:"read", Path:"|0", Err:0x9} ("read |0: bad file descriptor")
    ... STDOUT:
    STDERR:
```

0 files changed, 0 insertions, 0 deletions