gerrit-upload dryrun: output diff for already uploaded changes

[twmr]

If you upload a change that is already on gerrit it should be possible to somehow display the local changes (if there are any) between the local HEAD and the latest patchset of the change on gerrit.

If your local commit/change is in sync with the latest PS of the change on gerrit, gerrit-upload should output an error/warning.

To be performed steps:

• find the change with the changeid of the current change on the gerrit server and fetch (only fetch!) it
• if HEAD and FETCH_HEAD are identical, output a warning, otherwise
• diff HEAD against the FETCH_HEAD and display it in new buffer