`git nomad sync` should only show refs that have changed

[rraval]

Right now we print everything by delegating to git nomad ls.

Also, here's what git fetch origin puts out, look to mimic the format:

From github.com:rraval/repo
   909a311..b5f4895  master     -> origin/master

[rraval]

Summarizing the changed refs should be possible by parsing the output of git fetch --porcelain: https://git-scm.com/docs/git-fetch#_output

Or maybe just pass along unfiltered git fetch output?