R1:7626ec0ce10e

Correct an issue where "View Raw File" in Differential generated a file with overbroad permissions

Summary:
Via HackerOne. When you view a raw file in Differential, we currently generate a permanent file with default permissions. This may be incorrect: default permissions may be broader than the diff's permissions.

The other three methods of downloading/viewing raw files ("Download" in Diffusion and Differential, "View Raw" in Diffusion and Differential) already apply policies correctly and generate…