Fix diff submission for p4

Review Request #7439 — Created June 19, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.0.x
2f53fc3...

Reviewers

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.

Ran unit tests.

Able to submit a P4 diff.