Fix diff submission for p4
Fix diff submission for p4

Not all SCMTools (namely Perforce) will have new revision information
for files stored in `File.newInfo`. In Perforce, this is actually a
timestamp so attempting to parse it as a revision will fail.

Testing Done:
Ran unit tests.

Able to submit a P4 diff.

Bugs closed: 3887

Reviewed at https://reviews.reviewboard.org/r/7439/

reviewboard/reviewboard,chipx86/reviewboard,davidt/reviewboard,brennie/reviewboard,beol/reviewboard,davidt/reviewboard,beol/reviewboard,davidt/reviewboard,KnowNo/reviewboard,KnowNo/reviewboard,chipx86/reviewboard,chipx86/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,beol/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,KnowNo/reviewboard,beol/reviewboard,KnowNo/reviewboard,brennie/reviewboard,bkochendorfer/reviewboard,bkochendorfer/reviewboard,reviewboard/reviewboard,davidt/reviewboard,reviewboard/reviewboard