Prevent git p4 submit or sync if there are files checkout out in P4

[ludovicc]

It would be nice to run the following command before starting a p4 submit or sync command.
p4 opened -u $(git config git-p4.user) -C $p4repository

If there are files checked out in P4, then the submit/sync will fail miserably and the following synchronizations with P4 will be broken as well