Check author information to be valid before enableing
the commit button.

Task-number: QTCREATORBUG-2410

Details: Use a global model for nicknames that is shared by all dialogs to solve the updating  problems. Make p4 plugin submit work again. Force a submit prompt in case of shutdown or editor close (as opposed to user clickling on "Submit").

With many git commands it is safer to disambiguate files from refs by
putting '--' in front of the list of files.

Also corrected a margin in the git submit panel.