`ent sync` should report what changes got fetched & what changes will be pushed

Currently `ent sync` does:

1. `git fetch origin`
2. `git merge origin/entomologist-data`
3. `git push origin entomologist-data`

I think after (1), before (2), we can do some easy, partially useful things:

1. Show what we have locally that's not on the remote yet:
    `git log -p entomologist-data ^origin/entomologist-data`

2. Show what the remote has that we don't have locally:
    `git log -p origin/entomologist-data ^entomologist-data `

Eventually (maybe after issue 5fe71e27727f4243dc997e63b9a02971 and/or
issue fd81241f795333b64e7911cfb1b57c8f) we can display the changes in
a simpler way.