git-p4: parse marshal output "p4 -G" in p4 changes

git-p4: parse marshal output "p4 -G" in p4 changes

The option -G of p4 (python marshal output) gives more context about the
data being output. That's useful when using the command "change -o" as
we can distinguish between warning/error line and real change description.

This fixes the case where a p4 trigger for  "p4 change" is set and the command git-p4 submit is run.

Signed-off-by: Miguel Torroja <redacted>
Signed-off-by: Junio C Hamano <redacted>