Run git fetch before diffing the remote repository.

[bfabio]

For debops-update --dry-run to work properly, the remote needs to be
fetched beforehand.

[bfabio]

It's a little bit of a stretch on what a dry run should be, but there's no way (that I'm aware of) around it.

[drybjed]

Sounds like there's no way to avoid the fetch so I suppose that's OK. New commits won't be merged to existing branch.

[ypid]

@bfabio Note that the current implementation of reviewing could be exploited by our git hosting service (currently GitHub) as you do a fetch two times and the later one is used. Just something to note. It would be nice to see a more robust update mechanism implemented, one which is also tried with the debops-keyring (ref #164).