Perform additional checks at the beginning of gerrit-upload

[twmr]

Sometimes it happens that I'm not sure if the checked-out branch is in sync with gerrit, i.e., that all not-yet-submitted changes are on gerrit and are up2date.

It would be nice if e.g. magit-status showed in the Unmerged into section the following info for each commit

• was the change uploaded to gerrit
• has the change on gerrit already been submitted
• is the local version or the version on the server is newer

• gerrit-upload could skip the git push call if the version on the server is the same as the local version of the current branch/commit.