Fix for a "500: Encoding error" in the blame view

What does this MR do?

- The change enforces UTF-8 encoding on the source line data obtained from the Git.

Are there points in the code the reviewer needs to double check?

- The fix might not be enough to fix all encoding error issues in the blame view. There might be similar encoding issues if there are non-UTF-8 encoded names, for example

Why was this MR needed?

- This change fixes the "500: encoding error" that I get when I try to view the Gitlab source CHANGELOG file.

What are the relevant issue numbers / Feature requests?

- Gitlab issue #894: https://gitlab.com/gitlab-org/gitlab-ce/issues/894

See merge request !298