Wei Fu
6681cc4b94
ctr/snapshots/diff: don't show the media-type in output
By default, diff subcommand will print tar(.gz) on stdout. If we print
the media-type in stdout, the output will create invalid tar(.gz) data.
```
// before
➜ containerd git:(move_stdout_to_stderr) sudo ctr snapshot diff sha256:9f54eef412758095c8079ac465d494a2872e02e90bf1fb5f12a1641c0d1bb78b > /tmp/1.tar
➜ containerd git:(move_stdout_to_stderr) file /tmp/1.tar
/tmp/1.tar: data
// after change
➜ containerd git:(move_stdout_to_stderr) make bin/ctr
+ bin/ctr
➜ containerd git:(move_stdout_to_stderr) sudo bin/ctr snapshot diff sha256:9f54eef412758095c8079ac465d494a2872e02e90bf1fb5f12a1641c0d1bb78b > /tmp/2.tar
➜ containerd git:(move_stdout_to_stderr) file /tmp/2.tar
/tmp/2.tar: gzip compressed data, original size modulo 2^32 75155456
```
Signed-off-by: Wei Fu <fuweid89@gmail.com>