R1:df3162584eb7

Convert to custom encoding for diffusion blame views

Summary:
If a repository is configured with a custom encoding, it wasn't respected by DiffusionGitFileContentQuery making all views with
non-UTF8 characters fail. Check if we have a custom encoding and encode if any it set.

NOTE: This only works for Git repositories.

Test Plan: Browsed a repository with custom encoding before and after this patch.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T452

Differential…