builtins/commit: check for conflicting permissions options

This explicitly checks for commit command options asking for both
non-zero UID/GID and canonical permissions at the same time,
which are incompatible.

tests/basic-test.sh

@@ -437,7 +437,7 @@ echo "ok user checkout"
 $OSTREE commit ${COMMIT_ARGS} -b test2 -s "Another commit" --tree=ref=test2
 echo "ok commit from ref"
 
-$OSTREE commit ${COMMIT_ARGS} -b test2 -s "Another commit with modifier" --tree=ref=test2 --owner-uid=`id -u`
+$OSTREE commit ${COMMIT_ARGS} -b test2 -s "Another commit with modifier" --tree=ref=test2 --owner-uid=0
 echo "ok commit from ref with modifier"
 
 $OSTREE commit ${COMMIT_ARGS} -b trees/test2 -s 'ref with / in it' --tree=ref=test2