Fix remote history check - check if git fetch needs to be run

[tommy-mitchell]

Fixes #421.
Fixes #414.

If np detects that there are unfetched upstream changes (via git fetch --dry-run), errors with

Remote history differs. Please run `git fetch` and pull changes.

I'm not sure if it matters that it only checks origin. I guess if the remote name differs? Should the check be git fetch @{u} --dry-run?

According to the man page, origin can be omitted:

When no remote is specified, by default the origin remote will be used, unless there’s an upstream branch configured for the current branch.

[tommy-mitchell]

The tests pass locally, but I think it may be an issue with the execa stub. I'm still figuring it out.