Pull: print some data instead of newlines when output is not a terminal

This improves `docker pull` output when stdout is not a terminal which should fix #8140.
~~I had to change some logic for displaying json message output to fix some differences between handling terminal/non-terminal output, so some more testing should be best.~~

The pull output when running in cli should be the same, while new output when stdout is not a terminal looks like:
```
root@ff5a78202988:/go/src/github.com/docker/docker: ./bundles/1.2.0-dev/binary/docker pull busybox > test.txt
root@ff5a78202988:/go/src/github.com/docker/docker: cat test.txt
Pulling repository busybox
a9eb17255234: Pulling image (latest) from busybox
a9eb17255234: Pulling image (latest) from busybox, endpoint: https://registry-1.docker.io/v1/
a9eb17255234: Pulling dependent layers
511136ea3c5a: Download complete
42eed7f1bf2a: Download complete
120e218dd395: Download complete
a9eb17255234: Download complete
a9eb17255234: Download complete
```
@dqminh sorry but can you explain this change?