Use `p4 print -o` for getting the contents of perforce files.

Review Request #9469 — Created Jan. 4, 2018 and submitted — Latest diff uploaded


Review Board


The command which we use to get the contents of perforce files at a
specific revision, p4 print, tries to be smart about UTF-8 BOM
characters. If the output is to stdout, even if stdout isn't a terminal,
it will strip the BOM. Unfortunately, we had a difference between
RBTools, which uses p4 print -o, and Review Board, which uses
p4 print, so that the diffs produced by RBTools might not apply
cleanly to the files fetched by Review Board.

This change makes Review Board use p4 print -o so that the source file
will match the one used to create the diffs.

Ran unit tests.